Method of inspecting ununiformity of transparent material, apparatus therefor, and method of selecting transparent substrate

ABSTRACT

A laser beam L from a laser  2  is introduced from an introducing surface into a transparent substrate  1  by using mirrors  31  and  32.  The laser beam introduced through the transparent substrate  1  repeats a total reflection on the surfaces (main surfaces and end surfaces) of the transparent substrate  1  and enters a state in which the laser beam is almost confined in the substrate  1.  When an ununiform portion such as a scratch exists on the surface of the transparent substrate  1,  however, total reflecting conditions are not satisfied and the light leaks out of the ununiform portion. The leaked light is formed as an image on a CCD  6  by a lens system  7  and an image process is executed by an image processing apparatus  12.  In is a detected image, the ununiform portion in which the scratch or the like exists is brightly seen in a linear or a dot form in a black background, so that the ununiform portion such as a very fine scratch can be detected.

TECHNICAL FIELD

[0001] The present invention relates to a method of inspecting anoptical ununiformity (defect) of a transparent material such as a glasssubstrate serving as a transparent substrate for a photo mask or atransparent substrate for an information recording medium and anapparatus therefore. More particularly, the invention relates to amethod of inspecting an ununiformity of a transparent material and anapparatus therefore, whereby the ununiformity of the transparentmaterial can be detected at a high sensibility and a high speed by usingcharacteristics of a total reflection on the surface of the transparentmaterial and its apparatus.

BACKGROUND ART

[0002] In a manufacturing process of a semiconductor integrated circuit,a photo mask, or the like, a photolithography method is used to form afine pattern. For example, when the semiconductor integrated circuit ismanufactured, a pattern is transferred onto a transparent substratewhich was mirror-finished by polishing at a high precision by using aphoto mask whose pattern was formed by a transparent film (for example,a chromium film). As a method of inspecting the photo mask which can besaid as an original board of the pattern, as shown in a surface stateinspecting apparatus disclosed in Japanese Patent Application Laid-OpenNo. 58-162038 (1983), a method of converging light onto a fine region ona pattern surface and comparing a reflection output and transmissionoutput from the pattern surface has been known.

[0003] In recent years, however, in association with a realization ofhigh density of the pattern, in addition to the inspection for thepattern surface similar to the above method, a fine defect of thetransparent substrate itself which was mirror-finished by polishing at ahigh precision is also regarded as a target of the defect detection. Inthe above-mentioned method, since the light is converged onto the fineregion on the pattern surface, when an inspecting region extends over awide range, it is necessary to scan the light by using some means, along inspecting time is required in proportional to the area of theinspecting region, and a change in light quantity of the reflectionlight and transmission light for the pattern itself and transparentsubstrate is not large depending on the presence or absence of thedefect, so that it is difficult to apply the method to the detection ofthe fine defect on the transparent substrate.

[0004] Also with regard to a transparent substrate for an informationrecording medium, from the viewpoints of the formation of an under layerand a magnetic layer having a good crystallinity which are formed on thesurface of the transparent substrate, the low flotation of a magnetichead, and the like with the realization of high density recording, thetransparent substrate having the surface polished at a-high precision isrequired, so that the fine defect of the transparent substrate itself isalso set to a target of the defect detection. However, existing defectinspecting method and apparatus do not necessarily satisfy a request ofthe defect detection.

[0005] According to the invention, therefore, in order to solve theabove problems, it is an object to provide a method of inspecting anununiformity of a transparent material, whereby an optical ununiformityof the transparent material can be certainly detected at a highprecision and a high speed and an apparatus therefore.

[0006] Further, an object of the invention is to provide a method ofinspecting an ununiformity of a transparent material, whereby a desiredtransparent material can be immediately extracted and an apparatustherefore.

[0007]FIG. 1 is a schematic constructional diagram showing an embodimentof an apparatus for inspecting an ununiformity of a transparent materialaccording to the invention;

[0008]FIG. 2 is a side elevation view in which a part of the transparentsubstrate in FIG. 1 is enlarged;

[0009]FIG. 3 is a perspective view showing an example of a folder forholding the transparent substrate;

[0010]FIG. 4 is an image of a scratch on the surface of a glasssubstrate detected by the inspecting apparatus in FIG. 1;

[0011]FIG. 5 is a diagram showing a light intensity distribution in thewidth direction of the scratch obtained by light information of theimage of FIG. 4 by an image process;

[0012]FIG. 6 is an image when the scratch of FIG. 4 is observed by anoptical microscope (reflection·bright field);

[0013]FIG. 7 is a diagram showing a light intensity distribution in thewidth direction of the scratch obtained from light information of theimage of FIG. 6 by the image process;

[0014]FIG. 8 is a graph in which a detection sensibility of the methodaccording to the invention is compared with that of a conventionalmethod and which shows a relation between a signal-to-noise ratio(hereinbelow, referred to as an S/N ratio) and normalized exposing time;

[0015]FIG. 9 is a perspective view for explaining a simulation forobtaining a beam locus in the transparent substrate when a laser beam isintroduced into the transparent substrate along one side;

[0016]FIG. 10 is a diagram showing an example of a result according tothe simulation of FIG. 9;

[0017]FIG. 11 is a graph showing a relation between an incident angle tothe transparent substrate and the number of reflecting times on surfacesin FIG. 10;

[0018]FIG. 12 shows graphs of the beam loci showing states of thepropagation of the light in the transparent substrate obtained by thesimulation of FIG. 9;

[0019]FIG. 13 is a graph showing a relation between the incident angleto the transparent substrate and the number of reflecting times on thesurfaces in another example of the simulation of FIG. 9;

[0020]FIG. 14 is a graph showing the relation between the incident angleto the transparent substrate and the number of reflecting times onsurfaces in the other example of the simulation of FIG. 9;

[0021]FIG. 15 is a graph showing the relation between the incident angleto the transparent substrate and the number of reflecting times onsurfaces in the other example of the simulation of FIG. 9;

[0022]FIG. 16 shows graphs of beam loci showing results of simulating apropagation of the light when a laser beam is introduced to thetransparent substrate;

[0023]FIG. 17 shows diagrams showing images such as scratches of thetransparent substrate observed by an ununiformity inspection of theinvention;

[0024]FIG. 18 is a perspective view showing a state when laser beams areintroduced from two directions into the transparent substrate;

[0025]FIG. 19 is a schematic constructional diagram showing anembodiment in which the laser beam is introduced from a corner portionof the transparent substrate;

[0026]FIG. 20 is a perspective view showing an embodiment in which laserbeams are introduced from the same incident position in a plurality ofdifferent directions in the transparent substrate;

[0027]FIG. 21 is a flowchart showing an embodiment of an inspectingprocesses for separating good and bad transparent substrates from eachother;

[0028]FIG. 22 is a diagram for explaining the first embodiment in whicha method of selecting a transparent substrate according to the inventionis applied to a glass substrate for a photo mask;

[0029]FIG. 23 is a diagram showing a light intensity distribution in thewidth direction of a scratch obtained from light information of an imageof the scratch on the surface of the glass substrate detected by theinspection of the first embodiment in FIG. 22 by the image process;

[0030]FIG. 24 is a diagram for explaining the second embodiment in whichthe method of selecting the transparent substrate according to theinvention is applied to a glass substrate for a magnetic disk;

[0031]FIG. 25 is a diagram for explaining an inspecting method of thesecond embodiment of FIG. 24;

[0032]FIG. 26 shows diagrams showing another embodiment for theununiformity inspection of the invention;

[0033]FIG. 27 is a diagram showing a refraction of light on boundarysurfaces having different refractive indices;

[0034]FIG. 28 is a diagram used to obtain total reflecting conditions onthe surface of a transparent material; and

[0035]FIG. 29 is a diagram showing a region satisfying the totalreflecting conditions under which the light is confined in a rectangulartransparent material by a multiple total reflection.

DISCLOSURE OF THE INVENTION

[0036] According to the invention, there is provided a method ofinspecting an ununiformity of a transparent material by introducing alaser beam into the transparent material, wherein the transparentmaterial has: at least one pair of total reflective surfaces in whichthe laser beam introduced in the transparent material repeats the totalreflection and which face each other and at least one pair of turningsurfaces which are arranged so as to face each other in the progressingdirection of the laser beam that repeats the total reflection betweenthe total reflective surfaces and progresses and which totally reflectthe laser beam and return to the total reflective surfaces, when anoptical path in the transparent material is optically uniformed, thelaser beam is introduced in such a manner that the laser beam whichpropagates in the transparent material and impinges on the totalreflective surfaces and the turning surfaces of the transparent materialis propagated so as to totally reflect and repeat between at least thepair of turning surfaces and the laser beam is spread in an inspectingregion which is formed by the propagation and is surrounded by the totalreflective surfaces and the turning surfaces, and when an ununiformportion exists in the optical path of the laser beam which is introducedinto the transparent material and propagates, light that leaks out ofthe total reflective surfaces and/or the turning surfaces is detected,thereby inspecting an ununiformity of the transparent material.

[0037] When there is no ununiform portion such as a scratch on thesurface in the transparent material, the laser beam introduced throughthe transparent material repeats the total reflection on the surface andis confined in the transparent material (the laser beam is spread), sothat the light does not substantially leak to the outside (on the totalreflective surfaces and turning surfaces). When the ununiform portionexists in the transparent material, however, the total reflectingconditions are not satisfied and the light leaks out of the surface ofthe transparent material. That is, when the ununiform portion (defect)such as scratch, crack, and stain due to an adhering foreign matterexists on the surface of the transparent material, the light leaks outof the surfaces (total reflective surfaces and turning surfaces) astotal reflective surfaces so long as the optical path is uniform. Inaddition to the ununiformity of the surface of the transparent material,also with respect to the detection of a defect such as internal scratch,crack, or foreign matter such as bubbles or a defect of the glass inwhich a transmission is the same but a reflective index alone isdifferent, which fact is peculiar to the striae of the glass, light isout of the optical path (passage) where the light inherently passes ifit is uniform in the scratch or a portion where the refractive index isdifferent, the total reflecting conditions on the surface are notsatisfied, and the light leaks to the outside of the transparentmaterial, so that it can be detected.

[0038] As mentioned above, according to the inspecting method of theinvention, since the light is (substantially) confined in thetransparent material by using the geometrical and optical totalreflection serving as a physical critical phenomenon, responses toinspection light in the ununiform and uniform portions of thetransparent material as an inspecting object are also critical, so thatthe ununiformity appears as a dramatic contrast. That is, the inspectingmethod of the invention can be called as an inspecting method for thedefect (ununiformity) by a light confining method. A defect such as avery fine scratch of the transparent material is observed as light in ashielding case like a black box leaks out of a pinhole existing on thecase.

[0039] Incident conditions of the light for repeating the totalreflection on the surface of the transparent material to (substantially)confine the light in the transparent material are obtained as follows.The conditions to (substantially) confine the light by the multipletotal reflection in a rectangular transparent material like atransparent substrate will now be obtained hereinbelow.

[0040] Prior to the obtaining of the conditions to confine the light,first, as shown in FIG. 27, the direction of refraction light when thelight impinges on a medium such as glass having transparency of arefractive index nt from a medium such as air of a refractive index niis obtained (although a vector is expressed by a normal arrow in thediagram, a vector A is expressed as<A> in the document).

[0041] As shown in FIG. 27, a refraction beam <Lt> when an incident beam<Li> impinges on a boundary surface between the refractive indices niand nt is considered. A solvent vector which is perpendicular to theboundary surface and is directed to the side of an incident medium isset to <N> (unit vector). The vector <Lt> of the refraction beam existson the flat surface extended by the vectors <N> and <Li> and can beexpressed by a linear connection of <N> and <Li>. That is, it can beexpressed as follows.

<Lt>=α<Li>+β<N>  (1)

[0042] Where, α and β are coefficients. In order to simplify acalculation, when it is assumed that the vectors <Li>and <Lt>are set tounit vectors, the following equations are satisfied.

<Li>·<Li>=1, <Lt>·<Lt>=1  (2)

[0043] When a law of refraction (Snell's law) is applied to therefraction on the boundary surface, since an incident angle is θi and arefractive angle is θt,

sin θt=(ni/nt)sin θi  (3)

[0044] When θi and θt are expressed by vectors, $\begin{matrix}\begin{matrix}{< {Li} > {\cdot {< N>={{< {Li} > {{{\cdot {{< N >}}}{\cos \left( {\pi - {\theta \quad i}} \right)}}}}}}}} \\{{= {{- \cos}\quad \theta \quad i}}\quad}\end{matrix} & (4) \\\begin{matrix}{< {Lt} > {\cdot {< N>={{< {Lt} > {{{\cdot {{< N >}}}{\cos \left( {\pi - {\theta \quad t}} \right)}}}}}}}} \\{{= {{- \cos}\quad \theta \quad t}}\quad}\end{matrix} & (5)\end{matrix}$

[0045] An object which will now be obtained is the refraction beamvector <Lt>. When α and β of the equation (1) are determined, <Lt>isdecided. Therefore, when α and β are expressed by using ni, nt, and θifrom the equations (1) to (5),

α=ni/nt$\beta = {{- \left\{ {1 - {\left( {n\quad {i/n}\quad t} \right)^{2}\left( {1 - {\cos^{2}\theta \quad 1}} \right)}} \right\}^{\frac{1}{2}}} + {\left( {n\quad {i/n}\quad t} \right)\cos \quad \theta \quad i}}$

[0046] The direction of the refraction beam <Lt> to the incident beam<Li> is determined.

[0047] Subsequently, conditions under which the refraction beamintroduced in the predetermined direction as mentioned above repeats thetotal reflection on the surface of the rectangular transparent materialand is confined in the transparent material will now be obtained.

[0048] First, an xy plane is considered as a first boundary surface ofthe total reflection. As shown in FIG. 28, when it is assumed that anormal vector in the positive direction of a z axis is set to <Nz>=(0,0, 1) and an incident vector <L1> (unit vector)=(L1 x, L1 y, L1 z)impinges on the xy plane at the incident angle θi, the followingequation is satisfied.

<Li>·<Nz>=cos(π−θ1)

[0049] When the equation is expanded,

L1 z=−cos θ1

[0050] Since |<L1>|=1, the following equation is satisfied underconditions that when θ1≧θc.

L1 x ²+L1 y ²+cos²θ1=1  (6)

[0051] It is sufficient that the incident vector which can totallyreflect on the xy plane of the first boundary surface exists on theoutside of a circular cone in FIG. 28 obtained by rotating a straightline which forms a critical angle θc with the z axis around the z axis.The vectors which can satisfy the above-mentioned conditions infinitelyexist.

[0052] Since the vector reflects on the xy plane, the vector <L2> aftercompletion of the reflection is as follows. < L2 >  = (L1x, L1y, −L1z)   = (L1x, L1y, cos   θ  1)

[0053] Further, it is necessary to consider conditions under which thevector <L2> totally reflects on a second boundary surface. When it isassumed that the second boundary surface is set to an xz plane and thevector impinges on the xz plane at an incident angle θ2,

<L2>·<Ny>=cos(π−θ2)

[0054] Where, <Ny> denotes a unit vector (0, 1, 0) which is directed tothe positive direction of a y axis. When the above equation is expanded,

L1 y=−cos θ2

[0055] From the above equation and equation (6), it is necessary tosatisfy the following equation under conditions that when θ1, θ2≧θc.

L1 x ²=1−cos²θ1−cos²θ2  (7)

[0056] Since the vector reflects on the xz plane, a vector <L3> aftercompletion of the reflection is as follows.

<L3>=(+{1 cos²θ1−cos²2}^(1/2)·cos θ2, cos θ1)

[0057] When the vector <L3> totally reflects on a third boundarysurface, the confinement of the light is succeeded. When the thirdboundary surface is set to a yz plane and the vector impinges on the yzplane at an incident angle θ3,

<L3>·<Nx>=cos(π−θ3)

[0058] Where,Nx=(1, 0, 0). Therefore, the above equation is expanded toL3 x=−cos θ3, so that it is necessary to satisfy the expression of+{1−cos²θ1−cos²θ2}^(1/2)=−cos θ3, that is,

cos²θ1+cos²θ2+cos²θ3=1  (8)

[0059] sin(π/2)=(nt/ni)sin θc at the boundary angle θc. Therefore, whenni=1.00, nt=1.47, since sin θc=1/1.47, the boundary angle θc=42.90.

[0060] When θ1=θ2=θ3, according to the equation (8), 3 cos² θ1=1 andθ1=54°, such an equation of θ1≧54°>θc=42.9° is sufficiently satisfied. Amargin having a degree of freedom in the above-mentioned conditions canbe utilized as a degree of freedom which allow the total reflection tomore occur on a specific reflective surface. As for substrate glass, itis sufficient to select <L1> so as to reduce the reflection at the endsurface.

[0061] As mentioned above, the conditions under which the light isconfined by the multiple total reflection are as follows.

cos²θ1+cos²θ2+cos²θ3=1

cos θc≧cos θ1, cos θ2, cos θ3>0

[0062] When θc=42.9°, cos θc=0.73. Therefore, when a region where θ1,θ2, and θ3 are satisfied is shown in the diagram by using cos θ1, cosθ2, and cos θ3 as axes of coordinates, as shown in FIG. 29, a curvedsurface 60 serving as a surface of a sphere whose radius is equal to 1in a cube whose side is equal to 0.73 is the region to satisfy the totalreflecting conditions.

[0063] Although the total reflecting conditions with respect to therectangular transparent material like a transparent substrate has beenderived, it is also possible to decide the conditions of the incidentangle of the light by a method similar to the above in a general formwhich will be described hereinlater. The conditions of the incidentangle of the light which is introduced into the transparent material inthe embodiment, which will be described hereinlater, are derived on thebasis of the above.

[0064] In the method of inspecting the ununiformity of the invention,the transparent material can take any form of a square (rectangular)plate, a circular plate, an annular plate, a lens whose curved surfacehas a large curvature, a sphere, a polyhedron, a column, a cylinder, anda polyhedral column. When it is possible to realize a state where thelight repeats the reflection of the number of predetermined times ormore in at least the inspecting region of the transparent material andthe light is confined in the transparent material, the form of thetransparent material is not limited.

[0065] As mentioned above, in order to confine the light in thetransparent material, it is preferable that the transparent material hasat least one pair of total reflective surfaces In which the lightintroduced through the transparent material repeats the total reflectionand which face each other and at least one pair of turning surfaceswhich are arranged so as to face each other in the progressing directionof the laser beam that repeats the total reflection between the totalreflective surfaces and propagates and which totally reflect and returnthe laser beam to the total reflective surfaces. Particularly, theturning surfaces are important to confine the light in the transparentmaterial. The turning surfaces have to be provided so as to face eachother in the progressing direction of the light and it is necessary toprovide at least one pair. The reason is that if the light does notrepeat between the turning surfaces, the light cannot be confined.

[0066] As for the introduction of the laser beam, it is necessary tointroduce the laser beam in such a manner that the laser beam introducedthrough the transparent material is propagated so that light whichpropagates in the transparent material and impinges on the totalreflective surfaces and turning surfaces totally reflects and repeatsbetween at least the pair of turning surfaces and the laser beam isspread in an inspecting region which is formed by the propagation and issurrounded by the total reflective surfaces and the turning surfaces.That is, when the optical path in the transparent material is opticallyuniform, the laser beam is introduced in such a manner that the lightintroduced through the transparent material is propagated whilerepeating the total reflection between the total reflective surfaceswhich face each other, the light impinges on a turning surface (a) andtotally reflects, after that, the light further repeats the propagationin the transparent material and totally reflects between at least one ofturning surfaces (b), (c), . . . except for the turning surface (a), andthe light propagated in the turning surface (a) is again returned. Inthis case, the optical path in the transparent material is opticallyuniform. Even if there is no ununiform portion, the total reflection isrepeated between the total reflective surfaces and turning surfaces.Since there is no singular point that the light leaks on the totalreflective surfaces and turning surfaces (except for the region tointroduce the laser beam), the confinement of the light is(substantially) realized. In case of one pair of the turning surfaces,the light is confined on one certain plane of the transparent material.In case of a plurality of pairs of the turning surfaces, the confinementof the light is executed in almost the whole region of the transparentmaterial.

[0067] In order to (substantially) confine the light in the transparentmaterial, it is sufficient to introduce the laser beam so that such asingular point that the laser beam geometrically and optically leaksdoes not substantially exist on the total reflective surfaces and theturning surfaces. Since it is “geometrically and optically”, light suchas Rayleigh scattering light derived by an optical change due to afeature that is peculiar to the transparent material is not consideredas an introduced laser beam here. The reason why “such a singular pointthat the laser beam leaks does not substantially exist” is that a casewhere after the laser beam repeats the total reflection many timesbetween the total reflective surfaces and the turning surfaces, a veryslight laser beam does not satisfy the total reflecting conditions andleaks out of the transparent material due to an influence of a spreadangle of the laser beam itself. Therefore, it is “substantially” inconsideration of the case is considered. Since the leak light due to thespread angle of the laser beam itself leaks in the direction along thesurface of the transparent material, it is not detected by detectingmeans, so that it has no effect on a detecting sensitivity.

[0068] In order to confine the light in the transparent material, whenit is assumed that the refractive index of the transparent material to awavelength λ of the laser beam which is introduced is set to nt, therefractive index of the external medium which is come into contact withthe transparent material is set to ni, and an angle of the light whichimpinges on the total reflective surfaces and turning surfaces is set toθik (k denotes a position where the laser beam impinges on the totalreflective surfaces and turning surfaces after it is introduced throughthe transparent material and the incident positions are is sequentiallyset as k=1, 2, . . . ), it is sufficient to introduce the laser beam sothat θik is equal to a critical angle θ expressed as sin θ=ni/nt or morein the total reflective surfaces and turning surfaces.

[0069] As for the surface of the transparent material, with regard to aform in which it is difficult to confine the light by the totalreflection (for example, a form whose curved surface has a largecurvature), the ununiformity can be inspected by propagating the lightso as to repeat the total reflection on at least two pairs of surfaces(at least one pair of total reflective surface and at least one pair ofturning surfaces) which are provided on the outside of the transparentmaterial and which face each other. Specifically speaking, theinspection can be executed in such a manner that a transparent vesselhaving mirror-finished surfaces is used, a transparent material isinserted into a medium (a liquid or the like) in the vessel, which has arefractive index larger than that of an external medium of the vessel,and the laser beam is introduced so as to repeat the total reflection onthe external surface of the vessel and propagate.

[0070] It is preferable that by introducing the laser beam so that allof the laser beams totally reflect on at least the total reflectivesurfaces and turning surfaces on which the laser beam introduced throughthe transparent material first impinges, the light satisfying the totalreflecting conditions can be precisely introduced through thetransparent material. In a case other than the above, for example, whenscattered light propagates in the transparent material on the surface tointroduce the laser beam, since loci of (a plurality of) beams whichwill propagate in the transparent material cannot be expected, it isdifficult that almost all of the introduced laser beams which impingeson the total reflective surfaces and turning surfaces totally reflect,so that the confinement of the light as shown in the invention is notrealized.

[0071] It is desirable that an introducing surface to introduce thelaser beam is provided in a portion sandwiched by one certain totalreflective surface and at least one turning surface. As for theintroduction of the laser beam to the transparent material, the laserbeam can be introduced from the total reflective surface or turningsurface other than the introducing surface. In this case, however, as anentrance window to introduce the laser beam, an optical member made of amaterial having substantially the same refractive index as that of thetransparent material has to be attached by an adhesive agent or thelike, so that it takes much time. Since the optical member is attachedto the total reflective surfaces or turning surfaces serving asinspecting regions, the light propagated in the transparent materialdoes not satisfy the total reflecting conditions in the attached portionand the light leaks, so that the inspection cannot be substantiallyexecuted. Therefore, it is not preferable.

[0072] As mentioned above, when the introducing surface to introduce thelaser beam exists, as specific introducing means, the laser beam isintroduced so as to emit the introduced laser beam into only theintroducing surface and a surface in which an angle formed between thesurface and introducing surface is almost equivalent to an angle formedbetween the surface and total reflective surface, thereby realizing thelight confinement referred to in the present invention.

[0073] It is desirable that at least the introducing surface of thetransparent material to introduce the laser beam is mirror-polished. Thelaser beam is introduced so that the total reflection is repeated at thetotal reflective surfaces and turning surfaces and the light isconfined. When the introducing surface is mirror-polished, however, theintroduced laser beam is not influenced due to a diffusion ty theintroducing surface and is propagated as parallel light as it is.Consequently, all of the light which impinges on the total reflectivesurfaces and turning surfaces are totally reflected, so that responsesof the inspection light in the ununiform and uniform portions of thetransparent material become more critical and the contrast is improved.Preferably, it is desired that the whole surface (total reflectivesurfaces, turning surfaces, and introducing surfaces) of the transparentmaterial is mirror-polished.

[0074] It is assumed that the size of the total reflective surface isset to L, the width of introducing surface is set to d, the refractiveindex of the transparent material for the wavelength λ of the laser beamis set to nt, the refractive index of the external medium which is comeinto contact with the transparent material is set to ni, a beam diameterof the laser beam is set to φ, the angle of the light incident on thetotal reflective surfaces and the turning surfaces is set to θik (kdenotes the position where the laser beam first impinges on the totalreflective surfaces and turning surfaces after the laser beam isintroduced through the transparent substrate and the incident positionsare sequentially set as k=1, 2, . . . . Particularly, an angle of thelight which first impinges on the total reflective surface or turningsurface after the laser beam is introduced is set to θi.), and thenumber of reflecting times at the total reflective surfaces is set to m.When m is expressed by a function of L, d, nt(λ), ni, φ, and θ1, it ispreferable that conditions of at least one of L, d, nt(λ), ni, φ, and θ1are determined so that m is equal to a reference set value or more in arange where each θik is equal to the critical angle θ or more and thelaser beam is introduced from the introducing surface of the transparentmaterial.

[0075] When the light introduced through from the introducing surfacerepeats the propagation in the transparent material and the lightimpinges on the introducing surface again, since the light of thecritical angle θ or less impinges, the light leaks. Therefore, when itis desired that the total reflection is more performed in thetransparent material (when it is desired to increase m), it issufficient to reduce such a probability that the light leaks out of theintroducing surface. Actually, the beam locus is obtained by asimulation, the light introduced through the transparent materialrepeats the total reflection at the total reflective surfaces andturning surfaces and propagates, so that the width d of the introducingsurface that the number of reflecting times until the light leaks out ofthe introducing surface increases is determined. Specifically speaking,it is sufficient that the width d of the introducing surface is reduced.Although the width d of the introducing surface is also limited by abeam diameter of the laser beam and a process for the transparentmaterial, it is desirable that d is equal to 0.4 mm or less, preferably,0.2 mm or less. When it is extremely reduced (it is smaller than 0.1mm), since a break easily occurs on an interface between the totalreflective surfaces and the introducing surface and an interface betweenthe turning surfaces and the introducing surface, it is not preferable.

[0076] By selecting the refractive index nt (or wavelength λ of thelaser beam) of the transparent material, m (the number of reflectingtimes on the total reflective surfaces) can be adjusted. Specificallyspeaking, since there is a case where the quality of material of thetransparent material is limited in accordance with the use of thetransparent material, so that it is preferable to select the wavelengthX of the laser beam. The wavelength of the laser beam in which anabsorption to the transparent material is little is preferable. When theabsorption is large, since there is a possibility that not only adetecting sensitivity of the ununiformity decreases but also thetransparent material itself is broken, it is not preferable. Thewavelength of the laser beam also exerts an influence onto a resolutionof the ununiformity (scratch on the surface or the like). Since themaximum of the resolution of the ununiformity becomes the wavelength λof the laser beam, when it is desired to resolve and detect such a finedefect that the width of a scratch of a glass substrate for anelectronic device is equal to 1 μm or less as an image, the wavelengthof the laser beam is set to 1 μm or less.

[0077] When the transparent material has a certain specific form (forexample, when the total reflective surfaces are perpendicular to theturning surfaces or the like), since the angle at which the beamimpinges on each of the total reflective surfaces and turning surfaceshas a predetermined relation with the angle θ1 at which the laser beamfirst impinges on the total reflective surfaces after completion of theintroduction, m (the number of reflecting times on the total reflectivesurfaces) can be adjusted by properly adjusting the angle θ1. In fact,after the conditions of the transparent material (width d and refractiveindex nt of the introducing surface) and the conditions of the laserbeam (wavelength λ and beam diameter φ) are determined, θ1 is adjustedso as to be equal to the reference design value m which permits thetransparent material to be filled with light or more, and the laser beamis introduced. Generally, however, there is not a little variation insize (length or the like) of the total reflective surface depending on adifference between processing precisions. When the size of the totalreflectlve surface of the transparent material having the variation isgrasped every inspecting sample and the inspection is then performed, ittakes an extensive time, so that it is not practicable. Therefore, thelaser beam which is introduced through the transparent material can bedetected at a high sensitivity and a high speed by fluctuating theincident angle within a range where the total reflection is caused andimpinging the beam, so that the inspecting method having a high utilitycan be realized.

[0078] It is preferable that the total reflective surfaces and turningsurfaces of the transparent material have such a relation as to cross atright angles to each other. With such a construction, the introducedlaser beam easily enters a state where it repeats the total reflectionon the total reflective surfaces and turning surfaces and is confined inthe transparent material. In face, the inspection for the wide region ofthe transparent material can be simultaneously executed, so that a highspeed inspection can be realized. That is, the reason is that theincident angle of the light is the same on at least the pair of totalreflective surfaces on which the introduced laser beam repeats the totalreflection and, on at least the pair of turning surfaces as well, theincident angle of the light is the same, so that the light is propagatedso as to have a predetermined relation (when the incident angle of thelight on the total reflective surface is set to θ, the incident angle ofthe light on the turning surface is equal to 90°−θ).

[0079] In the inspecting region of the transparent material sandwichedby one certain pair of the total reflective surfaces and one certainpair of turning surfaces, the ununiformity in one certain plane in theinspecting region filled with the light due to such a fact that thelight is propagated in the transparent material is inspected and, afterthat, the ununiformity of the inspecting region is inspected byrelatively moving the plane of the inspection in the direction in whichthe inspecting region is filled with the light to the transparentmaterial, SO that the inspecting method can be simplified and it ispreferable.

[0080] Although the concept of the general light confinement of theinvention has been explained, when the invention is more specificallyembodied, there is provided a method of inspecting an ununiformity of atransparent material by introducing a laser beam through the transparentmaterial, wherein the surface of the transparent material has at leastone pair of main surfaces which are parallel to each other, at least onepair of end surfaces which intersect the main surfaces, and chamferportions sandwiched by the main surfaces and end surfaces, when anoptical path in the transparent material is optically uniform, a laserbeam is introduced in such a manner that light which propagates in thetransparent material and impinges on the main surfaces and the endsurfaces of the transparent material is propagated so as to totallyreflect and repeat between at least the pair of end surfaces and thelaser beam is spread in an inspecting region which is formed by thepropagation and is surrounded by the main surfaces, the end surfaces,and the chamfer portions, and when an ununiform portion exists in theoptical path of the light introduced and propagated in the transparentmaterial, the light that leaks out of the main surfaces and/or endsurfaces is detected, thereby inspecting the ununiformity of thetransparent material.

[0081] In this instance, the main surfaces, the end surfaces, and eachof the chamfer portions correspond to the foregoing total reflectivesurfaces, the turning surfaces, and the introducing surface. As arepresentative form having the main surfaces, end surfaces, and chamferportions, a square (rectangular) plate, a circular plate, an annularplate, or the like can be mentioned. In this case, the introduced laserbeam easily enters a state where the laser beam repeats the totalreflection on the main surfaces and end surfaces and is (substantially)confined in the transparent material. Actually, since a wide region ofthe transparent material can be simultaneously inspected and a highspeed inspection can be realized, it is preferable. That is, the reasonis as follows. Each incident angle of the light on the main surfaces onwhich the introduced laser beam repeats the total reflection is the sameand each incident angle of the light which impinges on the end surfacesis also the same. Since the light is propagated so that those incidentangles keep a predetermined relation (when it is assumed that theincident angle of the light which impinges on the main surfaces is setto θ, the incident angle of light which impinges on the end surfaces isequal to 90°−θ), the light confinement is (substantially) satisfied bymerely setting so that the incident angle to the main surfaces on whichthe light first impinges after it introduces through the transparentmaterial is larger than a critical point and the incident angle to theend surfaces is larger than the critical point.

[0082] As specific means for introducing the laser beam, the laser beamis introduced so that a singular point that the laser beam geometricallyand optically leaks out of the main surfaces and end surfaces does notsubstantially exist. Further, the laser beam is introduced so that theintroduced laser beam is emitted from only the chamfer portions.

[0083] It is also preferable that the transparent material serving as anobject of the ununiformity inspecting method is made of glass. Thequality of material of the transparent material is decided in accordancewith various uses. In case of glass, there are advantages that it ishard, a remarkably smooth surface can be obtained by mirror-polishing,light permeability is good, and the like.

[0084] When the transparent material is a glass substrate for anelectronic device, the ununiformity inspecting method exhibits theeffect still more. Since a glass substrate having a surface polished ata high precision is required in association with the realization of highdensity of a pattern in recent years, the ununiformity inspecting methodis effective in inspecting the ununiformity such as fine scratch orstriae of the substrate which becomes an adverse influence on aformation or an exposure of the pattern.

[0085] When the transparent material is a glass substrate for aninformation recording medium, the ununiformity inspecting methodexhibits the effect more and more. Since the transparent substratehaving the surface polished at a high precision is required inassociation with the realization of high density recording and lowflotation of a magnetic head in recent years, the ununiformityinspecting method is effective in inspecting the ununiformity such as ascratch on the substrate surface which becomes an adverse influence ontothe realization of high density recording and low flotation of themagnetic head.

[0086] Similar to the glass substrate for the electronic device, glasssubstrate for the information recording medium, or glass substrate for aliquid crystal display, when the material is formed so as to have mainsurfaces which are parallel to each other and end surfaces which areperpendicular to the main surfaces and so that introduced light repeatsthe total reflection and is confined in the transparent material,actually, a wide region of the transparent material can besimultaneously inspected, so that the inspection can be performed at ahigh speed.

[0087] An apparatus for inspecting an ununiformity of a transparentmaterial according to the invention is to embody the above inspectingmethod and is an apparatus for inspecting the ununiformity of thetransparent material by introducing a laser beam through the transparentmaterial, comprising: illuminating means (irradiating means) forintroducing a laser beam through the transparent material; and detectingmeans for detecting light which leaks out of the transparent material,wherein the transparent material has an introducing surface forintroducing the laser beam through the transparent material and at leasttwo pairs of surfaces on which the introduced laser beam repeats a totalreflection and which face each other, and the illuminating means isarranged so that a laser beam emitted from the illuminating means isintroduced from the introducing surface, when an optical path in thetransparent material is optically uniform, light which propagates in thetransparent material and impinges on the surfaces of the transparentmaterial is propagated so as to totally reflect and repeat at least onepair of surfaces among the above-mentioned surfaces, and the laser beamis spread in an inspecting region which is formed by the propagation andis surrounded by at least the two pairs of surfaces. With such aconstruction, the inspection of the ununiformity of the transparentmaterial can be automatically executed, an inspecting time can bereduced, and a reliability of the inspection can be improved.

[0088] In the inspecting apparatus, it is preferable that angleadjusting means for changing an incident angle of the laser beam to thetransparent material is provided for the illuminating means. The angleadjusting means adjusts the incident angle so that the laser beamintroduced through the transparent material repeats the total reflectionon the surface of the transparent material and the light is confined andis also used when the incident angle is fluctuated within a range wherethe total reflection occurs in order to absorb a variation in size dueto a difference between processing precisions of the transparentmaterial.

[0089] As representative angle adjusting means, a mirror can bementioned. The mirror is arranged between the illuminating means (forexample, a laser) and the transparent material and adjusts the incidentangle for the transparent material. In addition to the mirror, it isalso sufficient that angle adjusting means for changing an angle of theilluminating means to the transparent material is provided for theilluminating means itself or angle adjusting means is provided for afolder for holding the transparent material. Means for adjusting andfluctuating the incident angle by using an acousto-optical effect of anultrasonic beam such as an acousto-optical polariscope can be also used.

[0090] In the inspecting apparatus, it is preferable to provide moving(scanning) means for moving (scanning) an incident position of the laserbeam on the transparent material It is because the whole region of thetransparent material can be exhaustively and automatically inspected.For example, illuminating means such as a laser and angle adjustingmeans such as a mirror are mounted on the same table. By attaching adriving apparatus to the table, consequently, they can be sequentiallymoved along one side of the transparent material or by attaching adriving apparatus to the folder for holding the transparent material,the folder can be moved.

[0091] In the inspecting apparatus, it is preferable that thetransparent material and detecting means are integratedly and relativelymoved for the illuminating means. When a detecting region of thedetecting means is larger than the inspecting region, the transparentmaterial is relatively moved for the illuminating means, so that theinspection of the ununiformity can be performed. However, since theinspecting region is generally larger than the detecting region of thedetecting means, the transparent material and detecting means areintegratedly and relatively moved for the illuminating means. When theyare relatively moved, it is also possible that the illuminating means,namely, an illuminating optical system such as a laser is fixed and thetransparent material and/or detecting means are moved by using thedriving apparatus or the like, or the transparent material and/ordetecting means are fixed and the illuminating optical system such as alaser is moved.

[0092] In the inspecting apparatus, it is preferable that the detectingmeans has an image pickup camera having an image pickup device (CCD orthe like) and a lens for forming, as an image onto the image pickupcamera, the light which leaks out of the transparent material, and theimage pickup camera and/or lens are relatively moved in the depthdirection for the transparent material. By relatively moving the imagepickup camera and/or lens in the depth direction for the transparentmaterial, a focusing of the image pickup camera can be performed, sothat accurate information of the ununiformity (scratch on the surface,internal striae, bubbles, or the like) in the thickness direction of thetransparent material can be obtained. For example, the detecting meanssuch as image pickup camera and lens are fixed and the transparentmaterial, laser, and mirror are integratedly moved in the depthdirection of the detecting means. On the contrary, it is also possiblethat the transparent material, laser, and mirror are fixed and thedetecting means such as image pickup camera and lens are moved.

[0093] In the inspecting apparatus, it is desirable to providediscriminating means for discriminating the presence or absence, kind,and size of the ununiformity of the transparent material on the basis ofthe information detected by the detecting means.

[0094] With respect to the presence or absence, kind (scratch or crackon the surface portion, striae or foreign matter in the inside, or thelike), size (area, length, width, depth, region, or the like) of theununiformity which previously exists in the transparent material,(image) information of the light which leaks out of the surface, arelation (information) between (a light quantity of the leak light,luminance, intensity distribution, depth from the surface, or the like)are accumulated in a computer or the like and (image) information of thelight detected by the inspection is compared with the accumulatedinformation, so that the kind and size of the ununiformity of thetransparent material can be discriminated. As mentioned above, bydiscriminating the presence or absence, kind, and size of theununiformity of the transparent material, a desired transparent materialcan be extracted. Therefore, for example, a glass substrate having anununiformity that has an influence at the time of a formation of apattern or an exposure for a transferring object can be eliminatedbefore the following process after completion of the inspection or canbe returned to a re-polishing process, so that the productivity can beimproved.

[0095] In the ununiformity inspecting apparatus, it is preferable thatas for the information of the detected light, when the light detected bythe CCD is converted to an SIN ratio (10·log₁₀(S/N)) for normalizedexposing time of the CCD and is processed, the S/N ratio (10·log₁₀(S/N))is equal to 4.8 dB or more so long as the normalized exposing time isequal to 0.025 or more. In this instance, the normalized exposing timeis defined as (exposing time of the CCD)/(maximum exposing time of theCCD until a signal of a background reaches (20000/4095)×100 electrons).The foregoing normalized exposing time can be freely set due tomeasuring conditions or inspecting apparatus.

[0096] The reason is that when the S/N ratio is equal to 4.8 dB or more(an amount of the signal for the background is equal to a value that isthree times as high as that of noises), the value is an image processingpossible level which is generally known, so that the presence orabsence, kind, and size of the ununiformity of the transparent materialcan be accurately discriminated.

[0097] A method of selecting a transparent substrate according to theinvention is characterized by comprising the steps of: preparing atransparent substrate having an introducing surface to which a laserbeam is introduced, at least one pair of main surfaces on which theintroduced laser beam repeats a total reflection and which face eachother, and at least one pair of end surfaces provided so as to face eachother in the progressing direction of light; introducing the laser beamfrom the introducing surface in such a manner that when an optical pathin the transparent substrate is optically uniform, light whichpropagates in the transparent material and impinges on the main surfacesand the end surfaces of the transparent material is propagated so as tototally reflect and repeat between at least the pair of end surfaces andthe laser beam is spread in an inspecting region which is formed by thepropagation and is surrounded by the main surfaces and the end surfaces;detecting light which leaks out of the main surfaces and/or end surfaceswithout being totally reflected; and comparing the detected Informationwith information which has previously been stored and which correspondsto the presence or absence, kind, and size of an ununiformity existingin the transparent substrate, thereby selecting the transparentsubstrate.

BEST MODE FOR CARRYING OUT THE INVENTION

[0098] Embodiments of the invention will now be described hereinbelow byusing the drawings. FIG. 1 is a schematic constructional diagram showingan embodiment of an apparatus for inspecting an ununiformity of atransparent material according to the invention.

[0099] In FIG. 1, reference numeral 1 denotes a transparent substratemade of glass such as optical glass serving as an inspecting object. Asshown in FIG. 2, the transparent substrate 1 has parallel planes whichface each other and are constructed by main surfaces (surface and rearsurface) H and end surfaces (T planes and C planes as chamfer portions).Every plane is mirror-polished and, after that, it is cleaned. The mainsurfaces (surface and rear surface) have a role that the laser beamintroduced through the transparent substrate repeats a total reflectionand propagates and have a function as a total reflective surface. Theend surfaces (T planes) are arranged so as to face each other in theprogressing direction of the light, allow the light which repeated thetotal reflection on the main surfaces and propagated to repeat betweenmirror surfaces which face each other, and have a function as a turningsurface for turning the introduced and propagated light. The C planesare planes sandwiched by the main surfaces and end surfaces (T planes).Generally, in the C plane, since a fine scratch on the surface hardlybecomes a problem, the plane is not regarded as an object of theinspecting region. In the invention, it has a function as an introducingsurface for introducing the laser beam.

[0100] In this instance, every surface of the main surfaces (surface andrear surface) as total reflective surfaces, end surfaces (T planes)serving as turning surfaces, and C planes serving as introducingsurfaces is mirror polished. Particularly, such a fact that theintroducing surfaces are mirror polished has a sense in the lightconfinement of the invention. That is, by mirror-polishing theintroducing surfaces, the introduced laser beam propagates as almostparallel light without substantially being scattered, so that it ispossible to adjust almost all of the light which impinges on the mainsurfaces and end surfaces so as to be totally reflected. When theintroducing surfaces are not mirror-polished, the light is scattered onthe introducing surfaces, the light of a plurality of directionspropagates, and every beam locus cannot be expected, so that the lightconfinement of the invention is not satisfied. Although explanation ismade with respect to such a fact that the introducing surfaces aremirror-polished, so long as the laser beam can be introduced so that allof the laser beams are totally reflected on the main surfaces on whichthe laser beam that is introduced into the substrate at least firstimpinges, it is unnecessary to mirror-polish. For example, by coatingmatching oil or the like having the same refractive index as that of thesubstrate in order to form a pseudo mirror surface onto the introducingsurfaces as mirror surfaces, the light confinement of the invention isalso realized.

[0101] In order not to impede the total reflection on the surface and toeasily execute the inspection of the leak light, the transparentsubstrate 1 is horizontally held by a folder so as to reduce a contactportion as little as possible. FIG. 3 shows an example of the folder ofthe transparent substrate 1. A folder 20 has a rectangular framing formfor holding the transparent substrate 1. Receiving portions 21 forsupporting corner portions of the bottom surface of the transparentsubstrate 1 are formed at four corners on the inner side of the bottomof the folder 20. Spheres 22 for supporting the transparent substrate 1at dots so as to be come into contact with it are placed.

[0102] Illuminating means for introducing the laser beam to inspect anununiformity from the side surface of the transparent substrate 1 isprovided for the transparent substrate 1. The illuminating means has: alaser 2 as a light source for emitting an illuminating light; andmirrors 31 and 32 for allowing the laser beam to illuminate atpredetermined position and angle of the C plane. The laser 2 and mirrors31 and 32 are placed on a table 5 on which a driving apparatus 4 formoving the laser beam in parallel with the direction of a side la of thetransparent substrate 1. In the embodiment, in order to relatively movethe laser 2 and mirrors 31 and 32 for the transparent substrate 1 anddetecting means, the transparent substrate 1 and detecting means arefixed and the laser 2 and mirrors 31 and 32 can be integratedly moved bythe driving apparatus 4. (It is also sufficient that the drivingapparatus is attached to the transparent substrate 1 and detectingmeans, the laser 2 and mirrors 31 and 32 are fixed, so that thetransparent substrate 1 and detecting means can be integratedly moved.)In order to detect the ununiformity in the thickness direction of thetransparent substrate 1 by executing a focusing of a CCD when the laserbeam is introduced, the table (not shown) on which the laser 2 andmirrors 31 and 32 are placed can be moved in the directions of X, Y, andZ. The mirrors 31 and 32 are used for a fine adjustment of an angle orthe like. It is also sufficient to directly irradiate the laser beamonto the substrate 1 from the laser 2 without using the mirrors 31 and32. Incident angle adjusting means can be also provided so as to enablethe laser beam introduced through the transparent substrate 1 to impingeby fluctuating the incident angle in a range where the laser beam causesthe total reflection. As incident angle adjusting means, means having amechanism for automatically adjusting angles of the mirrors 31 and 32 bya control of a computer or the like or means such as an acousto-opticalpolariscope for allowing the incident angle by using an acousto-opticaleffect of an ultrasonic beam can be also used.

[0103] Detecting means for detecting the laser beam which leaks out ofthe transparent substrate 1 is provided above the transparent substrate1. The detecting means has a CCD 6 and a lens system (image formingoptical system) 7 for forming, as an image onto the CCD 6, the lightleaked out of the transparent substrate 1. An optical sensor fordetecting the light leaked out of the transparent substrata 1 is notlimited to the CCD but a photo-multiplier or the like can be also used.When a flexible laser beam is used as illuminating light, the leak lightfrom the substrate 1 is detected by eye observation and the detectingmeans can be also omitted. When the CCD is used as detecting means,there are a CCD of a full frame system having a mechanical shutterfunction and one of an interline system in which the mechanical shutteris not needed. In consideration of a durability, the CCD of theinterline system is preferable. In order to reduce noises, it is alsodesirable that the CCD has a forced radiating fan or a thermoelectriccooling function.

[0104] An image processing apparatus 12 constructed by a computer or thelike to process a detected image is connected to the CCD 6 through anA/D converter 11 for converting a detected analog signal into a digitalsignal.

[0105] The image processing apparatus 12 has a function for analyzing animage signal from the CCD 6 and displaying a form pattern, a lightquantity, an intensity distribution, or the like of the leak light dueto the ununiformity and a discriminating unit for discriminating thepresence or absence, kind (scratch or crack on the surface portion,striae, foreign matter in the inside, or the like), size (area, length,width, depth, region, or the like) of the ununiformity of thetransparent substrate 1. As information (form pattern, light quantity,luminance, intensity distribution, depth from the surface, or the likeof the leak light) of the leak light which corresponds to the kind orsize of the ununiformity existing in the transparent substrate,measurement values (basic data) and the like previously obtained bymeasurements have been inputted to a storage unit of the imageprocessing apparatus 12.

[0106] A specific inspecting method executed by using the inspectingapparatus in FIG. 1 will now be described. As an inspecting object, aglass substrate for a photo mask, whose size is 152.4×152.4×6.35 mm andin which a width of the C plane is 0.4 mm is inspected. As shown in FIG.2, the laser beam is impinged from the C plane of the glass substrate sothat an incident angle θi to the main surface on which the laser beamfirst impinges after it introduces through the glass substrate is largerthan a critical angle θc and an incident angle (90°−θi) to the endsurface of the glass substrate is larger than the critical angle θc.Since a refractive index of the glass substrate is 1.47 and the criticalangle θc is about 42.9°, the incident angle θi is set to 44.1°. That is,the method of introducing is characterized with respect to a point thatthe laser beam is introduced so that a singular point that the laserbeam (geometrically and optically) leaks does not exist on the mainsurfaces and end surfaces of the glass substrate and the introducedlaser beam is emitted only from the C planes. As a laser, an He—Ne laseris used and a laser beam in which a beam diameter is 0.5 mm, a spreadangle of the beam is 1 mrad, a laser power is 0.5 mW, and a wavelengthis 543 nm is irradiated.

[0107] Since the substrate which is used this time has a form in whichall of the main surfaces as total reflective surfaces and the endsurfaces as turning surfaces have such a relation as to cross at rightangles to each other, the incident angles of the light which impinges onthe main surfaces are the same, the incident angles of the light whichimpinges on the end surfaces are the same, and the light propagates sothat those angles have a predetermined relation (when the incident angleof the light which impinges on the main surface is set to θi, theincident angle of the light which impinges on the end surfaces is equalto 90°−θi). Therefore, by merely setting so that the incident angle θito the main surface on which the light first impinges after itintroduces through the glass substrate and the incident angle 90°−θi tothe end surface are larger than the critical angle θc, the lightconfinement is realized. In case of a normal form (for example, a formin which the main surfaces and end surfaces do not have such a relationas to cross at right angles to each other, it becomes complicatedslightly. In this instance, when it is assumed that the refractive indexof the glass substrate for a wavelength λ of the laser beam which isintroduced is set to nt, a refractive index ni of an external medium(air) which is come into contact with the glass substrate is set to 1,an angle at which the light impinges on the main surfaces and endsurfaces of the glass substrate is θik (k denotes a position where thelaser beam impinges on the main surfaces and end surfaces after it isintroduced through the glass substrate and the incident positions k aresequentially set as k=1, 2, . . . ), the light confinement is notrealized unless the laser beam is introduced so that each θik is equalto the critical angle θ shown by sin θ=ni/nt or more. As mentionedabove, the case where the main surfaces and end surfaces have such arelation as to cross at right angles to each other is more effectiveagainst the light confinement.

[0108] As shown in FIG. 1, the laser beam introduced through thetransparent substrate (glass substrate) 1 by the illuminating meansrepeats the total reflection on the main surfaces and end surfaces ofthe substrate 1 and enters a state where the light is almost confined inthe substrate 1. The state where the light is almost confined means thatthe introduced laser beam propagates in the transparent substrate andrepeats the total reflection and continues to propagate in thetransparent substrate until the light impinges on the chamfer portionserving as an introducing surface, namely, so long as it impinges on themain surfaces and end surfaces. Therefore, the laser beam incident inthe Y direction is scanned everywhere so as to be spread in a region(measuring region) of a cross section (YZ cross section) obtained bycutting the substrate 1 in the Y direction by the propagation due to thetotal reflection of the light itself.

[0109] As mentioned above, the laser beam introduced through the glasssubstrate repeats the total reflection on the main surfaces and endsurfaces of the glass substrate and enters the state in which the beamis almost confined in the glass substrate. However, when there is ascratch or the like on the glass surface due to a mixture of a foreignmatter upon polishing, the total reflecting conditions are notsatisfied, so that the light leaks out of a portion of the scratch. Withrespect to a defect of the glass in which a transmission is the same buta refractive index alone is different, which fact is peculiar to thestriae of the glass as well, the light is out of an inherent orbit(optical path) at a portion where the refractive index is different andleaks to the outside of the substrate 1 without being totally reflectedon the main surfaces and end surfaces. The leak light is detected by thedetecting means.

[0110] Due to the light irradiation in the above one cross section, theinspection of one line as observed from the main surface side can beexecuted. The inspecting process is executed by moving the table 5 inthe direction (X direction) of one side la of the substrate 1 by thedriving means 4, so that the ununiformity of the whole region of thesubstrate 1 can be inspected. That is, it is a method of inspecting anununiformity in such a manner that in an inspecting region of asubstrate sandwiched by main surfaces as one certain pair of totalreflective surfaces and end surfaces as one certain pair of turningsurfaces of the substrate, an ununiformity (defect) on one certain planein the inspecting region filled with light by propagating the light inthe substrate is inspected and, after that, the inspected plane isrelatively moved in the direction which permits the inspecting region tobe filled with the light for the substrate.

[0111] A result detected by the inspecting apparatus is shown in FIGS. 4and 5. FIG. 4 shows an image of a scratch on the surface of the glasssubstrate detected by the CCD 6. FIG. 5 is a graph obtained byperforming an image process to information of the light detected by theCCD on one certain side in the width direction of the scratch by acomputer via an A/D converter (analog-digital converter). The CCD usedat that time is a CCD of the interline system (having no mechanicalshutter) in which a thermoelectric cooling function is mounted and inwhich the number of elements is 1300×1030, a detecting area is 8.71×6.90mm, and a saturation amount of the CCD is 20000 electrons. As for themeasuring conditions, an exposing time of the CCD is set to 200 msec.

[0112] An X axis of FIG. 5 denotes a coordinate in the width directionof the scratch. A Y axis denotes an intensity of the detected light. Aunit of a scale of the X axis is a pixel. In the inspecting apparatus,since an objective lens of ×50 (50 times) and an image forming lens of×0.45 (0.45 times) are used, one pixel is 6.7 [μm]/(50×0.45), namely, itcorresponds to about 0.3 μm. (6.7 μm indicates a size of one pixel ofthe CCD.) The light intensity is resolved to 12 bits (4096:1) and onescale denotes (20000/4095)·Y(Y: scale) electrons. As will be obviouslyfrom FIG. 5, the intensity of the light leaked out of the scratch is(20000/4095)×4095=20000 electrons as a peak value that exceeds aallowance value of the CCD and the intensity of light in a region otherthan the scratch is equal to 0. As mentioned above, in an image which isdetected by the detecting means, the ununiform portion in which thescratch or the like exists is brightly seen in a linear or a dot form ina black background. The ununiform portion such as a scratch can beclearly discriminated from the obtained image. In this instance, whenthe propagation due to the total reflection in the glass substrate isconsidered, there is no factor that the light in the substrate is weakenduring the propagation except for a very slight absorption in theuniform portion, so that the light continues to propagate in the glasssubstrate. Therefore, almost all of the irradiated light concentrate onthe ununiform portion as a result. The ununiform portion sharply appearsat a very clear contrast. Therefore, a minute scratch or the like can bedetected at a high sensibility.

[0113]FIG. 6 shows an image of a scratch similar to that of FIG. 4observed by an optical microscope (reflection·bright field). FIG. 7 is agraph showing the image to which the image process similar to that inFIG. 5 was performed. As will be understood from FIGS. 6 and 7, signalsof the scratch are buried by signals of the background and the scratchcannot be detected by the method. When the scratches of FIGS. 4 and 6are observed by an atomic force microscope (AFM), each of them isconfirmed as a scratch in which the width is 0.13 μm and the depth is0.0013 μm.

[0114] In the above embodiment, when it is desired to confirm theincident angle θi of the laser beam to the transparent substrate 1, forexample, as shown in FIG. 2, so long as an wedged optical member 8 isarranged on the substrate surface via matching oil or the like, theincident angle θi can be obtained from a refractive index γ of lightwhich is emitted from the optical member 8 or an apex angle of theoptical member 8. When something like the optical member 8 is used as anentrance window to introduce the inspecting light into the substrate,the light can be also introduced from portions other than the chamferportions (C planes) of the substrate.

[0115]FIG. 8 shows a relation between an S/N ratio and a normalizedexposing time when an arbitrary scratch is observed and the imageprocess similar to the above is performed in order to clarify adifference between a case where the ununiformity is detected by theoptical microscope in a conventional normal illumination and a casewhere the ununiformity is detected by the inspecting method of thepresent invention. In this instance, the normalized exposing time isdefined as (exposing time of the CCD)/(maximum exposing time of the CCDuntil the signals in the background reach (20000/4095)×100 electrons)and the S/N ratio is set to 10·log₁₀(S/N). As will be obviouslyunderstood from FIG. 8, in the inspecting method in the conventionalnormal illumination, even when the normalized exposing time is extended,an maximum is at most 3 dB. In the inspecting method of the presentinvention, the S/N ratio exceeds 30 dB as a result. In the inspectingmethod of the invention, the SIN ratio is limited up to 36 dB asmaximum. It is because the saturation amount of the CCD camera islimited. It is considered that an extremely high S/N ratio exceeding 36dB is actually obtained. (It is considered that the reason why the S/Nratio in the normal illumination indicates a minus value is that thesignal of the scratch is buried by noises.) According to the inspectingmethod of the present invention, therefore, the S/N ratio exceeds 4.8 dB(the signal for the background is equal to an amount that is three timesas much as that of noises) which is generally known as an imageprocessing possible level by far, so that the presence or absence, kind,and size of the ununiformity in the transparent material can beaccurately discriminated.

[0116] In the above embodiment, the incident angle θi is set to 44.1°.The optimum incident angle at which the total reflection is repeatedmore can be easily selected by simulations shown as follows. The resultsobtained by simulating a situation in which the light propagates in thetransparent substrate will now be explained hereinbelow.

[0117] First, a calculation result when the light is propagated alongone side (in the y axial direction) of the transparent substrate 1 asshown in FIG. 9 will now be described. In the simulations, the dimensionof the transparent substrate 1 is 152.4×152.4×6.35 mm that is the sameas that of the glass substrate for the photo mask in the foregoingembodiment. The width of C plane is set to 0.4 mm. The refractive indexof the transparent substrate 1 is set to 1.47 that is a refractive indexof quartz glass and the refractive index around the transparentsubstrate 1 is set to 1.00 that is a refractive index of air. A vector(unit vector) indicative of the direction of the beam which impinges onthe C plane (which forms an angle of 45° against the main surface and Tplane) obtained by chamfering the transparent substrate 1 is set to(0.0000000, 0.6864532, −0.7271740). FIG. 10 shows results of thesimulations.

[0118] In FIG. 10, the incident angle is the incident angle to thesurface (main surface) on which the beam first impinges after it entersthe substrate 1. The incident angle is changed every 0.05 degree ofangle. A z coordinate upon emitting indicates a z coordinates when thebeam is emitted from the transparent substrate 1 and the bottom surfaceof the substrate 1 is set to z=0.

[0119]FIG. 11 is a graph showing the number of reflecting times on thesurfaces at respective incident angles. As will be understood from FIG.11, it is sufficient that the incident angle at which the number ofreflecting times on the surfaces increases is selected in accordancewith the form or the like of the transparent substrate. It is alsosufficient that the incident angle of the light which is introduced isfluctuated.

[0120]FIG. 12 shows states of the propagation of the light in thesubstrate in case of the incident angle of 43.35°. FIGS. 12(1), (2), and(3) show states in which the number of reflecting times on the surfacesare set to 50, 250, and 661 times (upon emitting), respectively. In thissimulation, in order to simplify the calculation, the propagation isperformed in the only region in the cross section of one plane (yzplane). As shown in FIG. 12, it will be understood that the light beamrepeats the total reflection and propagates so as to fill the region. Ina manner similar to the simulation, when the parallel light isintroduced in the one side (y direction) of the transparent substrate,in order to impinge the illuminating light on the whole region of thetransparent substrate, it is sufficient that the light is scanned alonganother side (x direction) by using a mirror or the like or the lightspread in a slit shape in the x direction is introduced from the Cplane.

[0121] The simulation results when the width of C plane, refractiveindex of the glass substrate (corresponding to the wavelength of thelaser beam) in the simulation of FIG. 9 are changed are shown in FIGS.13, 14, and 15. The simulation is executed under the conditions similarto those of the simulation executed in FIG. 9 other than a fact that therefractive index of the glass substrate is set to 1.46 (corresponding tothe wavelength of the laser beam that is equal to 543 nm) and the widthof C plane is changed to 0.2 mm (FIG. 18), 0.4 mm (FIG. 14), and 0.8 mm(FIG. 15).

[0122] As will be understood from FIGS. 13 to 15, as the width of Cplane increases, the number of reflecting times on the surfacesdecreases. The reason is as follows. When the laser beam introduced fromthe C plane propagates in the transparent substrate and again impingeson the C plane, the beam leaks without being totally reflected becausethe light incident to the C plane is impinged at an angle that issmaller than the critical angle θ. Consequently, a probability that thelight propagated in the transparent substrate impinges on the C plane israised by increasing the width of C plane. Therefore, in order toincrease the number of reflecting times on the surfaces in thetransparent substrate, it is sufficient to reduce the width of C plane.In case of the glass substrate (152.4×152.4×6.35 mm) for the photo maskused at this time, since the transparent substrate is sufficientlyfilled with the light so long as the number of reflecting times on thesurfaces is about 300 times, it is preferable that the width of C planeis equal to 0.4 mm or less.

[0123] When FIGS. 11 and 14 in each of which the width of C plane is 0.4mm are compared, the critical angle for satisfying the total reflectingconditions is changed by changing the refractive index (or wavelength ofthe laser beam (because the refractive index of the transparentsubstrate is decided by the wavelength of the laser beam)) of thetransparent substrate, SO that the number of reflecting times on thesurfaces can be adjusted. As for the critical angle to satisfy the totalreflecting conditions, as a difference between the refractive index ofthe transparent substrate and that of the external medium (for example,the air) of the transparent substrate is larger, a degree of freedom ofthe critical angle increases. In accordance with the above, the numberof reflecting times on the surfaces also increases. In fact, however,there is a case where the material of the transparent substrate islimited depending on the use. Therefore, ordinarily, the number ofreflecting times on the surfaces can be adjusted by properly selectingthe wavelength of the laser beam. As a wavelength of the laser beam, thewavelength in which the absorption for the transparent substrate islittle is preferable. Since it is influenced on the resolution of theununiformity, the wavelength of the laser beam is selected inconsideration of the following points.

[0124]FIG. 16 shows loci of the light beam in the general directionintroduced through the transparent substrate, which is not parallel tothe one side similar to the above simulation. In the simulation, avector indicative of the direction of the light beam which impinges onthe C plane of the transparent substrate 1 is set to (0.6924, 0.3823,−0.6117) and the conditions other than the above are the same as thoseof the simulation (FIG. 9). As shown in the diagram, the introducedlight beam repeats the total reflection in the transparent substrate 1and is substantially confined in the substrate, so that the lightpropagates in the whole area of the substrate. Therefore, even when thescanning of the illuminating light is not executed at all, the wholerange of the transparent substrate as an inspecting region can beinspected in a lump at a high speed.

[0125] In consideration of the simplification of the inspecting method,the case where one certain surface of the transparent material isdecided as shown in the above embodiment, the incident angle satisfyingthe total reflecting conditions is determined in the surface, the lightis introduced, and after that, the incident position of the light ismoved in accordance with the form of the transparent material is morepreferable than the case where three dimensional directional vectors (x,y, z) of the incident light are set so that the introduced light coversthe whole region in the transparent material and the light is introducedfrom a certain point on the substrate, because the inspecting method canbe simplified. When the transparent material is the substrate having thesurfaces which face each other, it is particularly effective.

[0126] In the above embodiment, the example in which the laser beam wasintroduced from one side la of the transparent substrate 1 has beenmentioned. The invention is not limited to the above but it is alsosufficient that the inspection is executed by introducing the light fromthe direction of one side 1 b or from two directions of the sides 1 aand 1 b. When the inspection is executed by introducing the light fromthe two directions of the sides 1 a and 1 b, it is effective to detect adefect having directionality or the like and the inspection at a higherprecision can be performed, so that it is preferable.

[0127] As mentioned above, the present invention is extremely effectivewith respect to the detection of the defect having directionality forthe light, which can be detected because it glints in a specificirradiating direction but cannot be detected because it does not glintin the other irradiating directions, which fact is peculiar to a scratchon glass. The reason is as follows. Since the light is substantiallyconfined in the inspecting object made of the transparent material bygeometrically and optically repeating the total reflection, theirradiated light is deviated from an inherent orbit only in theununiform portion of the inspecting object and leaks out of theinspecting object from the geometrical and optical viewpoints. Even ifthe only ununiform portion is the defect having directionality for thelight, the ununiform portion is illuminated from various directions inthe process to repeating the total reflection. In the conventionalmethod, since the light is converged in order to raise the contrast andthe light from the one direction is irradiated, even if the defect has arelatively large size, the defect having the directionality can behardly detected.

[0128] Also with regard to the defect of the glass in which thetransmittance is the same but the refractive index alone is different,which fact is peculiar to the striae of the glass, the light is deviatedfrom the inherent orbit in a portion where the refractive index isdifferent and leaks out of the inspecting object, so that the defect canbe detected. In the conventional method of detecting a light amount suchas reflection output or transmission output of the converged light,however, the detection is impossible in principle.

[0129] By using the inspecting method of the above-mentioned embodiment,the glass substrate having the defect can be rapidly and properlyexcluded, so that the productivity of the glass substrate can beimproved. By again precisely mirror-polishing and cleaning the glasssubstrate having the defect such as a scratch on the surface, it can beformed as a glass substrate for the photo mask in a range of aspecification.

[0130] The above inspecting method is used in the inspecting processafter the manufacturing process of the glass substrate as a transparentsubstrate for the photo mask. There is a variation in size (length orthe like) of the glass substrate depending on a difference of theprocessing precision (ordinarily, a allowance of the transparentsubstrate for the photo mask has about ±0.4 mm in length and about ±0.1mm in thickness). Therefore, when variant sizes of the glass substratesare grasped one by one, the optimum total reflecting conditions for eachof the glass substrates are obtained, and they are inspected, it takes along time and it is not practical. Because, when such an inspectingmethod that the accurate dimensions of the glass substrates aremeasured, the incident conditions under which the total reflection moreoccurs are grasped, and after that the laser beam is impinged, an extratime as much as {(time for measuring the dimensions of the glasssubstrates)+(simulating time)}×(the number of inspecting substrates) isrequired before the inspection.

[0131] In this case, by fluctuating the incident angle of the laser beamwhich is introduced through the glass substrate in a range where thetotal reflection is done on the main surfaces (surface and rear surface)and end surfaces (other than the C planes) and the light is reflectedbetween at least the pair of end surfaces and impinging the laser beam,even when there is a variation in dimensions of the glass substrates,the optical ununiformity of the glass substrate can be detected at ahigh sensitivity and a high speed, so that there are providedununiformity inspecting method and apparatus of a high utility.

[0132] That is, when the optical path in the transparent material isoptically uniform, the incident angle is changed in a range where thetotal reflection may occur on the surface of the transparent materialand the light is introduced in the transparent material. Consequently,even when there is a variation in dimensions of the transparentmaterials and the optimum total reflecting conditions for thetransparent materials slightly differ, the incident light of apredetermined direction is not introduced but the light having differentincident angles is introduced and propagates various paths while beingtotally reflected, so that the light is spread up to the corners of thetransparent material without leaking.

[0133] As fluctuating the incident angle for the substrate, in a mannersimilar to the angle adjusting means in FIG. 1, a machine which isconnected to a computer or the like and which can automatically controlthe angle is attached to the mirror, an angle adjusting mechanism isprovided for the laser itself or the folder for holding the substrate,or means such as an acousto-optical polariscope for fluctuating theincident angle by using the acousto-optical effect of the ultrasonicbeam can be also used. As an incident angle to the substrate, forexample, in case of the above-mentioned glass substrate(152.4×152.4×6.35 mm) for the photo mask made of quartz glass, it isdesirable that the incident angle θi of the laser beam is successivelychanged in a range of 45.0° to 44.0°.

[0134] The introduction of the laser beam to the transparent material isperformed on the basis of information of the transparent material.Consequently, when a plurality of transparent materials are inspected,particularly, the inspection can be efficiently executed.

[0135] In this instance, the information of the transparent materialindicates the relative positional relation between the transparentmaterial and illuminating means, the state of the surface of thetransparent material (whether it has been mirror-polished), or the like.The information regarding the relative positional relation between thetransparent material and the illuminating means is necessary to properlyintroduce the light from the illuminating means into a predeterminedposition of the transparent material. When the surface is not in themirror state, it is difficult to detect the ununiformity of thetransparent material. Therefore, the information regarding the surfacestate of the transparent material can be used when such a substrate ispreviously eliminated (as necessary, it is returned to the precedingprocess (polishing or the like)).

[0136] By providing position detecting means for detecting the relativepositional relation between the transparent material and theilluminating means and transmitting means for transmitting informationobtained by the position detecting means to the illuminating means, whena plurality of materials are inspected, the inspection can beefficiently executed. In this instance, the position detecting meansindicates a distance measuring device (laser scan measuring system,laser interference measuring device, or the like) by using the laserbeam. The transmitting means indicates a computer for fetching data fromthe position detecting means and feeding back to the illuminating means,the angle adjusting means, moving means for moving the incident positionof the introduction light, and the like. It is also sufficient toprovide an apparatus such as TV camera or CCD image pickup device imagesensor for observing the surface state of the transparent material and,for example, eliminating the inspecting object, for example, which isnot mirror-polished.

[0137] In the above embodiment, it is desirable to providediscriminating means for discriminating the presence or absence, kind,and size of the ununiformity of the transparent material on the basis ofthe information of the light detected by the detecting means.

[0138] The relation (information) between the presence or absence, kind(scratch or crack of the surface portion, internal striae or foreignmatter), and size (area, length, width, depth, region, or the like) ofthe ununiformity previously existing in the transparent material and theinformation of the light which leaks out of the surface (light amount,luminance, intensity distribution, depth from the surface of the leaklight) has been stored in the computer or the like. By comparing theinformation of the light detected by the inspection and the storedinformation, the presence or absence, kind, and size of the ununiformityof the transparent material can be discriminated. By discriminating thepresence or absence, kind, and size of the ununiformity of thetransparent material, a desired transparent material can be immediatelyextracted. Consequently, the glass substrate having the ununiformitywhich has an influence on the time of formation of the pattern or theexposure for a transferring object can be eliminated before the nextprocess after the inspection or can be also returned to the re-polishingprocess, so that the productivity can be improved.

[0139] The discriminating method will now be specifically explained byusing the inspecting apparatus in FIG. 1. The light leaked out of thesubstrate 1 is formed as an image on the surface of the CCD 6 of the CCDcamera by the lens system 7. As mentioned above, for a period of timewhile the laser irradiating region of one line is scanned onto the wholesurface of the main surface of the substrate 1, the shutter of the CCDcamera is opened as It is and image data of the whole surface of themain surface of the substrate 1 is accumulated. The image data fetchedin the CCD camera is converted to a digital signal by the A/D converter11, the converted signal is inputted to the image processing apparatus12 and is stored into the storage unit, and an image analysis isperformed by the discriminating unit. In the discriminating unit, bycomparing the image data of the light detected by the inspection withbasic data of the image information which has previously been inputtedin the storage unit, the presence or absence, kind, and size of thetransparent substrate 1 are discriminated. The moving amount(information of the irradiating position of the substrate 1) of thetable 5 or the like is inputted from a laser interferometer (not shown)or the like to the image processing apparatus 12. From the image data ofthe CCD camera and position data of the substrate 1, which kind and sizeof ununiform portion exists on which position (x, y) of the substrate 1is obtained.

[0140] When the ununiform portion exists in the irradiating region ofthe substrate 1, the ununiform portion (and its periphery) is brightlyseen in a dot form. When they are enlarged by the optical microscope,images as shown in FIG. 17 are observed (the images in FIG. 17 are shownby reversing bright and dark of images that are actually observed). Alinear image 41 as shown in FIG. 17(a) is a scratch on the surface ofthe substrate 1. As a representative size, the length is 30 μm, thewidth is 0.2 μm, and the depth is 0.002 μm (the size of such a finescratch was measured by the atomic force microscope). Many collectedimages 42 as shown in FIG. 17(b) are caused by striae or foreign matterssuch as gasses in the substrate 1. As a representative size, thediameter is about 1 mm. As mentioned above, since the pattern or size ofthe image differs depending on the ununiformity existing in thesubstrate 1, the kind of ununiformity can be discriminated. Further,since the images 42 by the striae are seen so as to glimmer as comparedwith the image 41 by the scratch, it can be also discriminated from theluminance or intensity distribution of the image.

[0141] The size of the ununiform portion can be discriminated from alight amount of the detected light. Further, whether the position wherethe ununiformity exists is located on the surface portion of thesubstrate 1 (scratch or clack) or in the substrate 1 (striae or foreignmatter) can be discriminated from a location (depth) where a focal pointis obtained by focusing on bright dotty portions of the substrate 1 bythe optical microscope. As for the inspection of the ununiformity, inorder to realize a high speed process, it is desirable that whether thebright dotty leak light exists on the surface of the substrate 1 isfirst inspected and, with respect to only the substrate 1 in which theleak light was detected, bright dotty portions is further inspected byenlarging or the like by the optical microscope.

[0142] In order to discriminate the ununiformity, when the light isintroduced from the different incident positions and differentdirections (two directions) to the substrate 1, accurate information ofthe leak light can be obtained in even case of the ununiformity (defect)having directionality. Consequently, since the presence or absence,kind, and size of the ununiformity can be accurately discriminated, itis preferable. As a method of introducing the light, as shown in FIG.18, a laser beam L1 is introduced in the direction (X direction) of theside la of the transparent substrate 1 and a laser beam L2 is introducedin the direction (Y direction) of the side lb at the same time or it isalso sufficient that the laser beams of the different directions areintroduced every direction (X and Y directions or the like) to thesubstrate 1, thereby inspecting.

[0143] As shown in FIG. 19, when the ununiformity is inspected byintroducing a laser beam L from the corner portion of the transparentsubstrate 1 by using, for example, a laser 13 and mirrors 14 and 15, thekind and size of the ununiformity can be discriminated by an imageforming optical system 16, a CCD camera 17, and an image processingapparatus 18 in a manner similar to the above.

[0144] In the above embodiment, although the example in which theinspection was performed by introducing the laser beam in only the ydirection of the substrate 1 has been mentioned, when the laser beam isintroduced from the different incident positions and differentdirections to the substrate 1, it is not necessarily that the incidentpositions are made different. For example, as shown in FIG. 20, evenwhen the laser beams L1 and L2 are introduced from the same incidentposition of the substrate 1 to a plurality of different directions asobserved from the main surface side of the substrate 1, the effect ofthe invention that the ununiformity having directionality can becertainly detected can be accomplished.

[0145] Although the introduction of the laser beam to the transparentsubstrate has been performed from the chamfer portion as a C plane inthe above embodiment, it is also possible to introduce the beam fromplanes other than the chamfer portions. In this case, it is sufficientthat as an entrance window to introduce the laser beam, an opticalmember made of a material having substantially the same refractive indexas that of the transparent substrate is attached by an adhesive agent orthe like. In order to simplify the inspecting method and inspect theununiformity of the whole region in the transparent substrate byrepeating the total reflection more, it is desirable to introduce thelaser beam from the chamfer portion as a C plane. The reason is thatwhen the optical member having the entrance window to introduce thelaser beam is attached, the light propagated in the transparentsubstrate does not satisfy the total reflecting conditions in a portionof the optical member, so that the light leaks out of the portion. It ispreferable that the chamfer portion is mirror-polished. As the width ofchamfer portion is smaller, it is more desirable. It is better to setthe width to be equal to 0.4 mm or less, more preferably, 0.2 mm orless. Even if it is set to be extremely small (smaller than 0.1 mm),since a defect occurs upon mirror-polishing, it is not preferable.

[0146] A method of selecting the transparent substrate which can be usedfor various uses by using the inspecting method and inspecting apparatusof the invention will now be explained with reference to the drawings.FIG. 21 is a diagram showing a flowchart for the inspecting process toselecting the transparent substrate.

[0147] A specific selecting method performed by using the inspectingapparatus in FIG. 1 will now be described with reference to theflowchart for the inspecting process of FIG. 21.

[0148] Decision·Alignment of the Inspecting Region

[0149] As a transparent substrate 1 serving as an inspecting target, aglass substrate for a photo mask made of quartz glass in which both mainsurfaces, end surfaces, and chamfer surfaces are mirror-polished, whosesize is 152.4×152.4×6.35 mm, and in which the width of each C plane isequal to 0.4 mm is prepared.

[0150] The glass substrate is conveyed by conveying means (not shown)until it is come into contact with a stage guide pin (not shown) fixedto a certain reference position of the inspecting apparatus, therebypositioning the glass substrate (process 1). At that time, an origin andcoordinates in the glass substrate are decided.

[0151] The inspecting region is specified on the basis of the previouslydecided coordinates. The inspecting region does not always coincide witha measuring visual field of the CCD. Therefore, when they don't coincidewith each other, the measuring region is divided into (A1, A2, A3, B1,B2, B3, . . . ) in correspondence to the visual field of the CCD (FIG.22) (process 2). In this instance, the divided measuring regions A1, A2,A3, B1, B2, B3, . . . coincide with the measuring visual field of theCCD. The CCD used for the measurement is the CCD of the interline system(having no mechanical shutter) having a thermoelectric cooling functionis mounted, in which the number of elements is 1300×10³⁵ and a detectingarea is 8.71×6.90 mm. The measuring visual field is measured at amagnification of 0.7 time.

[0152] The incident position and the incident angle of the laser beamare adjusted so that the laser beam propagates in the inspecting area(process 3). As for the incident position and incident angle of thelaser beam, information of the glass substrate is obtained by positiondetecting means (not shown) for detecting the relative positionalrelation between the transparent substrate and the laser and the laserbeam is introduced by adjusting the mirrors and table so that the laserbeam can be accurately introduced through the glass substrates havingdifferent sizes. As for the incident angle, since the refractive indexof the glass substrate is 1.46 and the critical angle θc is about 43.2°,the incident angle θi is set to 45.0°.

[0153] When the laser beam is introduced into the glass substrate, theincident angle of the laser beam is fluctuated in a range where the beamrepeats the total reflection and propagates (process 4). In theprocedure for inspecting a plurality of glass substrates, even if therespective glass substrates have a little variation in sizes dependingon a difference between the processing precisions, since the beam locuspropagating in the glass substrate is deviated little by little bychanging the incident angle of the light, the process is executed inorder to absorb the variation of processing precisions of the glasssubstrate and inspect the ununiformity of the glass substrate. Theprocess is also executed for an image matching of the CCD in the nextprocess. As means for fluctuating the incident angle, means having afunction for automatically adjusting the angle of the mirror by acontrol of a computer or the like or means such as an acousto-opticalpolariscope for fluctuating the incident angle by using theacousto-optical effect of the ultrasonic beam can be also used. As afluctuation of the incident angle, the incident angle θi is successivelychanged in a range of 45.0° to 44.0° so as to satisfy the totalreflection.

[0154] In order to precisely discriminate the light which leaks out ofthe transparent substrate, namely, the ununiformity, the focusing of theCCD image is executed (process 5). The focusing is executed byintegratedly moving the laser and the mirrors in an a axial direction(direction to the lens and CCD). It is also sufficient that the glasssubstrate, mirrors, and laser are fixed and the lens and CCD areintegratedly moved in a z axial direction.

[0155] Inspection of Ununiformity in Inspecting Region

[0156] As shown by enlarging one part in FIG. 22, the laser beam L isintroduced from the chamfer plane serving as an introducing surface sothat the laser beam L which passes certain coordinates (A1X1, A1Y1) ofthe measuring region A1 as one of the divided regions and is parallel tothe y axial direction propagates, the incident angle is changed in arange where the laser beam repeats the total reflection and propagatesin the glass substrate (of 45.0° to 44.0°), thereby inspecting theununiformity. The similar scan is performed so that the laser beam L ismoved in the x axial direction, the inspection of the ununiformity isperformed until the laser beam passes coordinates (A1X1, A1Y1) at theend portion of the measuring region A1, so that the inspection of theununiformity in the measuring region A1 is finished (process 6). Theexposure of the CCD is executed from the start to the completion of theinspection of the ununiformity in the measuring region A1. As for theinspection of the ununiformity in the measuring region A1, it is alsosufficient that the laser beam is impinged so that the laser beam whichpasses (A1X1, A1Y1) and is parallel to the x axial direction propagatesand it is moved in the y axial direction. It is also sufficient tocombine and introduce the laser beams in the two directions which crossat right angles to each other. As mentioned above, in case ofintroducing the light of a plurality of different directions as observedfrom the main surface side, the light is irradiated into the glasssubstrate from the plurality of directions. Even when the ununiformity(defect) having directionality exists, it can be accurately detected.

[0157] Image Process

[0158] In the process 6, information (analog signal) of the light whichwas detected by the CCD and which leaked out of the glass substrate isconverted into a digital signal by the A/D converter in order to executean image process by information accumulating means such as a computer.The information of the light converted into the digital signal isaccumulated by the information accumulating means such as a computer,the intensity of the light as shown in FIG. 28 is resolved to 12 bits(4096:1), and the image process is performed (process 7). The Y axis inFIG. 23 denotes the intensity of the light and one scale denotes(20000/4095)·Y(Y:scale) electrons.

[0159] Discrimination of Allowance of Ununiformity

[0160] As a result of the image process of the process 7, theununiformity existing in the glass substrate is discriminated as ascratch on the substrate surface. When it is compared with(20000/4095)×200 electrons which has previously been set as an allowancedesign value of the scratch (when the background is (20000/4095)×100electrons or less), since it exceeds the allowance design value, it isdiscriminated that the glass substrate is bad (process 8).

[0161] In the embodiment, since the scratch exceeding the allowancerange is found on the substrate surface in the inspecting area A1, theinspection for the ununiformity is not executed in the next inspectingarea A2 but the process is shifted to the re-polishing and cleaningprocess for the substrate. When no ununiformity is found in theinspecting region A1, the previously divided inspecting regions A2, A3,B1, B2, . . . and the processes 6 to 8 (according to circumstances,processes 2 to 8) are repetitively executed. When it is discriminatedthat the ununiformity is equal to the allowance design value or lower inthe whole inspecting region, the glass substrate for the photo mask isselected as good.

[0162] By using the selecting method of the embodiment, the glasssubstrate having the defect can be rapidly and properly eliminated, sothat the producibility of the glass substrate can be improved. By againprecisely mirror-polishing and cleaning the glass substrate having thedefect, it can be formed as a glass substrate for the photo mask whichlies in the range of the specification.

[0163]FIG. 24 is the second embodiment in which the method of selectingthe transparent substrate of the invention is applied to a glasssubstrate for a magnetic disk. The explanation for the processesoverlapped in the first embodiment in which the method is applied to theglass substrate for the photo mask is omitted.

[0164] As a transparent substrate 1 serving as an inspecting target, adisc-shaped glass substrate for magnetic disk made of quartz glass inwhich both main surfaces (H), inner rim edge surface (T1 plane) andouter rim edge surface (T2 plane), and chamfer surfaces (C planes) aremirror-polished and in which the diameter is 95 mm (3.5 inchesφ), thethickness is 0.8 mm, and a diameter of a circular hole on a centerportion is 20 mm φ is prepared.

[0165] As inspecting regions, as shown in FIG. 24, the region on themain surface of the transparent substrate is divided into the measuringregions A1, A2, A3, . . . from the inner rim side to the outer rim side.The inspection regarding the ununiformity is executed every dividedmeasuring region.

[0166] The inspection for the ununiformity is executed in such a mannerthat the laser beam L is introduced from the outer rim edge surface-ofthe disc-shaped glass substrate 1 in the direction of the center (O) ofthe disc, the light is confined in one plane in the radial direction (rdirection) including the outer rim edge surface and the inner rim edgesurface (so that the total reflection is repeated in both the mainsurfaces of the transparent substrate and the light is returned betweenthe inner rim and outer rim edge surfaces), the disc is rotated by adriving apparatus (not shown) for rotating the disc, and the laser beamL is moved in the direction to the rim (θ direction) of the disc.Specifically explaining with reference to FIG. 25, the laser beam L isintroduced to the chamfer surface (C plane) as an introducing surface bya laser 25 and mirrors 26 and 27 so that the laser beam L which passescertain coordinates (A1 r 1, A1θ1) of the measuring region A1 divided asshown in FIG. 25 and is parallel to the r direction is propagated, theincident angle is changed in a range (45.0° to 44.0°) where the beamrepeats the total reflection and propagates in the disc-shaped glasssubstrate 1, thereby inspecting the ununiformity. The disc-shaped glasssubstrate 1 is rotated, the same scan is performed by moving the laserbeam in the θ direction, and when the inspection for the ununiformity inthe region which passes the coordinates (A1 r 1, A1θX) of the measuringregion A1 is finished, the inspection of the ununiformity in themeasuring region A1 is completed. In the inspection for theununiformity, it is also sufficient that the laser beam is impinged fromthe inner rim edge surface of the disc or from both of the inner rim andouter rim edge surfaces.

[0167] In a manner similar to the first embodiment, the image processand allowance discrimination for the ununiformity are executed.Consequently, since the defect exceeding the allowance range is notfound in the inspecting region A1, the inspecting region is changed tothe inspecting regions A2, A3, B1, B2, . . . and the inspection for theununiformity similar to that in the inspecting region A1 is executed.Although the inspection for the ununiformity is performed in the wholeregion of the disc-shaped glass substrate, no defect exceeding theallowance range is found, so that it is discriminated as good.

[0168] In a manner similar to the first embodiment, when the defectexceeding the allowance range is found in the certain inspecting region,it is discriminated as bad and the process can be also shifted to there-polishing and cleaning processes for the substrate without executingthe inspection for the ununiformity in the following inspecting region.

[0169] In the above-mentioned inspecting method and selecting method, asunnecessary light which leaks out of the ununiformity (defect) of thetransparent material and which allows the contrast of the light to bedecreased, Rayleigh scattering light which scatters due to a microscopicfluctuation of a density that is peculiar to the transparent material orthe like exists. In order to reduce the unnecessary light, at least twolight having different wavelengths are introduced into the transparentmaterial or light having a specific polarization is introduced, so thatthe contrast of the detection light for the ununiformity is improved andthe detection can be realized at a higher sensitivity and a higherprecision. In the former case of introducing at least the two lighthaving different wavelengths, since the Rayleigh scattering lightbecomes light having a color obtained by mixing the light havingdifferent wavelengths, the scattering light can be eliminated byapplying a (color) filter for absorbing or reflecting a wavelength areaof the mixed light between the transparent material and the detectingmeans. In the latter case of introducing the light having the specificpolarization, the light becomes light having peculiar polarizingcharacteristics in a peculiar polarizing state by the Rayleighscattering. By using a difference between the polarizing characteristicsand those of light which leaks by the ununiformity, a polarizing devicesuch as polar screen, polarizing plate, or polarizing prism is placedbetween the transparent material and the detecting means, so that theRayleigh scattering light can be effectively eliminated.

[0170] As unnecessary light which decreases the contrast of the lightthat leaks out of the ununiformity (defect) of the transparent material,there is stray light derived in such a manner that light which was notintroduced in the transparent material is reflected on the surface ofthe transparent material and is impinged on the detecting system fordetecting detection light for the ununiformity. In order to reduce thestray light, the introduction light is reduced in correspondence to thesize of the introducing surface of the transparent material on which thelaser beam is impinged by converging the beam by the optical system suchas a lens and the introducing surface is shaped into a concave crosssection so that the converged laser beam is introduced as almostparallel light into the transparent material from the introducingsurface. Consequently, the stray light can be reduced, the contrast ofthe detection light for the ununiformity can be increased, and thedetection can be executed at a high sensitivity and a high precision.

[0171] As another factor to reduce the contrast of the light which leaksout of the ununiformity (defect) of the transparent material, as shownin FIG. 26 (in the diagram, (1) is a perspective view and (2) is a crosssectional view), there is a case where the light shielding material is arectangular plate having main surfaces, end surfaces, and chamfersurfaces, when the ununiformity is inspected by introducing the laserbeam L from the introducing surface (chamfer surface (C plane)), thelight leaks out of the chamfer surfaces other than the introducingsurface and becomes stray light. In this case, portions between thechamfer surfaces other than the chamfer surface which face each other inthe progressing direction of the light to introduce the light areconnected by an introducing light 50 formed by binding a plurality ofoptical fibers arranged in the surface direction of the chamfersurfaces, thereby enabling the light which leaks out of the chamfersurfaces to be again introduced through the transparent material.Therefore, the stray light is reduced, the introduced laser beam can bemore effectively concentrated to the ununiform portion, so that thecontrast is increased and the detection can be realized at a highsensitivity and a high speed.

[0172] In the embodiment in the inspecting method and the first andsecond embodiments in the selecting method, the transparent substratemade of glass has been mentioned as a transparent material having themirror-polished surfaces. It is not limited to glass but any material,for example, optical plastic such as acrylic resin or optical crystalsuch as quartz through which the inspection light can transmit can beused.

[0173] In the embodiment in the inspecting method and the first andsecond embodiments in the selecting method, the example in which thewhole surface of the transparent substrate was mirror-polished has beenmentioned. It is not limited to the above but a transparent substratehaving a part or whole of the surface which is not mirror-polished canbe also used. For example, in case of the glass substrate as a glasssubstrate for the photo mask, there is a case where the end surfacesother than the main surfaces, in which the pattern is not formed, arenot mirror-polished. In case of the glass substrate for the magneticdisk, there is a case where the inner rim and outer rim edge surfaces inwhich a film such as a magnetic layer is not formed are notmirror-finished. In this case, by coating a liquid such as a matchingoil onto the surfaces which are not mirror-finished, the surfaces lookas if they are mirror-polished (free surface of liquid, pseudo mirrorsurface), so that the ununiform portion can be inspected by theinspecting method and inspecting apparatus of the invention. Moreparticularly, when it is desired that the ununiform portion alone(striae, bubbles, foreign matter, or the like) existing in thetransparent material is inspected at a stage where the mirror-finishingis not executed, it is effective.

[0174] As a liquid which is coated in order to form the pseudo mirrorsurface, a matching oil or a sealing agent used for optical parts, ormasking agent for scratch of the glass can be mentioned. The liquidcoated on the surface of the transparent substrate can be in a liquidstate as it is or in a solid state of jelly, a hard film, or the likeafter completion of the coating. As a coating method of the liquid, anymethod such as brush coating (a brush or a sponge-like material issoaked in liquid, thereby coating), spray coating, or spin coating whichcan smoothly coat on the surface of the transparent material can beused. In this case, the proper method is selected in accordance with theliquid which is used or coating surface.

[0175] When the refractive index of the transparent material issubstantially the same as that of the liquid, the liquid-coated surfacein a mirror state optically and substantially becomes the surface of thetransparent material, so that the light introduced through thetransparent material can be certainly totally reflected and be returnedinto the inside. Specifically speaking, since quartz glass (refractiveindex is 1.46) or the like is often used as a transparent substrate, asa liquid whose refractive index is approximate to the above and whichcan be easily handled, canada balsam (refractive index is 1.52),Enterannew (trade name, refractive index is 1.49), diiodemethane(ethylene iodide, refractive index is 1.74), cedar oil (refractive indexis 1.52), liquid paraffin (refractive index is 1.48), Aquatex (tradename, refractive index is 1.4), glycerol (refractive index is 1.46), andthe like can be mentioned.

[0176] As for a water insoluble material such as canada balsam orEnterannew, the refractive index and viscosity can be adjusted by addingorganic solution such as xylene. As for a water soluble material such asglycerol or Aquatex, the refractive index and viscosity can be adjustedby adding water. As a masking agent for scratch of the glass, there isemulsion composition in which polyorganosiloxane andpolydiorganosiloxane are main components disclosed in Japanese PatentApplication Laid-Open Publication No. 6-4496 (1994) or the like.

[0177] As an inspection in the case where the whole surface of thetransparent substrate is not mirror-finished, for example, there is acase where the ununiformity alone (striae, bubbles, foreign matter, orthe like) in the transparent substrate is inspected. In this case, whenthe ununiform portion exists In the inside, it results in fatal defect.For example, in case of the glass substrate for a phase shift mask,since the bad one can be eliminated by inspecting at a stage before themirror-finishing, the costs of manufacturing can be also held down.

[0178] The form of the transparent material is not limited to the square(rectangular) or circular substrate but the transparent material cantake any form of a block form, a sphere, a column, a cylinder, apolyhedron, and a form having curved surfaces. Particularly, when asubstrate having surfaces which face each other, more particularly, asubstrate having at least two pairs of parallel planes which face eachother (for example, square (rectangular) or circular cone) is used assuch an above-mentioned transparent material, the introduced lightrepeats the total reflection and easily enters a state in which thelight is confined in the substrate. In fact, the inspection for the wideregion of the transparent material can be simultaneously executed andthe inspection can be executed at a high speed. As a substrate, further,the inspection can be applied to various kinds of substrates such asglass substrate for the electronic device (for the photo mask (phaseshift mask)), glass substrate for the liquid crystal display, or glasssubstrate for information recording (such as magnetic disk or opticaldisk) Since the glass substrate for information recording isdisc-shaped, in case of actually inspecting, the laser beam is impingedfrom the polished outer rim or inner rim edge surface (for example,chamfer portion). When the inspection for both the surfaces of thesubstrate is needed, it is also sufficient that the detecting means areprovided on the upper and lower sides of the substrate, respectively,and both of the substrate surfaces are inspected in a lump.

[0179] In the above embodiment, the gas laser (He—Ne laser) has beenused as a laser. It is not limited to the above but a laser of a visiblearea such as a semiconductor laser or, so long as it is absorbed to thetransparent substrate by little, an excimer laser of an ultravioletarea, an Nd-YAG laser of an infrared area, a CO₂ laser, or the like canbe used as a light source for the inspection. Particularly, in case ofusing the laser of the ultraviolet area (for example, a higher harmonicwave of the excimer laser, a YAG laser, or the like), since the foreignmatter or the like adhered on the substrate surface can be eliminateddue to the operation such as evaporating or transpiring, it ispreferable.

[0180] In the above embodiment, the example in which the angle adjustingmeans for changing the incident angle for the substrate was attached tothe mirrors located between the laser and the substrate has beenmentioned. So long as the incident angle of the laser beam for thesubstrate can be changed, any construction can be used. It is alsosufficient that the angle adjusting means is provided for the laseritself or it is provided for the folder for supporting the substrate. Asfor the introduction of the laser beam, it is also sufficient that thelight is introduced by using not mirrors in the embodiment but anoptical fiber. At that time, it is sufficient that an emitting edgeportion of the optical fiber is moved along each side of the substrateby using a guide or the like or an fluctuation is applied to the side ofthe emitting edge portion of the optical fiber, thereby fluctuating theincident angle.

[0181] As described in detail, according to the invention, since thelaser beam is confined in the transparent material by using the totalreflection as a physical critical phenomenon, the responses to theinspection light in the ununiform and uniform portions of thetransparent material become critical and the ununiformity appears as avery clear contrast, so that the ununiformity such as a fine scratch canbe detected at a high sensitivity and it can be also detected at a highprecision and a high speed. Further, not only the ununiformity on thesurface of the transparent material but also the defect such as internaldamage or striae can be also detected.

[0182] The presence or absence, kind, and size of the ununiformity ofthe transparent material are discriminated on the basis of theinformation of the light leaked out of the surface of the transparentmaterial, so that a desired transparent material can be immediatelyextracted and the producibility of the transparent material can beimproved.

1. A method of inspecting an ununiformity of a transparent material byintroducing a laser beam through the transparent material, characterizedin that said transparent material has: at least one pair of totalreflective surfaces in which the laser beam introduced through thetransparent material repeats a total reflection and which face eachother; and at least one pair of turning surfaces which are arranged soas to face each other in the progressing direction of the laser beamthat repeats the total reflection between said total reflective surfacesand progresses and which totally reflect said laser beam and return tosaid total reflective surfaces, when an optical path in said transparentmaterial is uniform, the laser beam is introduced in such a manner thatthe laser beam which propagates in the transparent material and impingeson said total reflective surfaces and said turning surfaces ispropagated so as to totally reflect, repeat between at least said onepair of turning surfaces, and is spread in an inspecting region which isformed by the propagation and is surrounded by said total reflectivesurfaces and said turning surfaces, when an ununiform portion exists inthe optical path of the laser beam which is introduced and propagated insaid transparent material, light which leaks out of said totalreflective surface and/or said returning surfaces is detected,transparent material according to any one of claims 1 to 3,characterized in that the laser beam is introduced so that all of thelaser beams totally reflect on said total reflective surfaces or saidturning surfaces on which the laser beam that is geometrically andoptically introduced through said transparent material impinges at leastfirst.
 5. The method of inspecting the ununiformity of the transparentmaterial according to any one of claims 1 to 4, characterized in that anintroducing surface to introduce the laser beam is provided in a portionsandwiched by one of said total reflective surfaces and at least one ofsaid turning surfaces.
 6. The method of inspecting the ununiformity ofthe transparent material according to claim 5, characterized in that thelaser beam is introduced so as to emit the introduced laser beam intoonly said introducing surface and a surface in which an angle formedbetween said surface and said introducing surface is almost equivalentto an angle formed between the surface and said total reflectivesurface.
 7. The method of inspecting the ununiformity of the transparentmaterial according to claim 5 or 6, characterized in that at least saidintroducing surface is mirror-polished.
 8. The method of inspecting theununiformity of the transparent material according to any one of claims5 to 7, characterized in that when it is assumed that the size of saidtotal reflective surface is set to L, the width of said introducingsurface is set to d, the refractive index of the transparent materialfor the wavelength λ of said laser beam is set to nt, the refractiveindex of the external medium which is come into contact with thetransparent material is set to ni, a beam diameter of said laser beam isset to φ, the angle of the light incident on said total reflectivesurfaces and said turning surfaces is set to θik (k denotes the positionwhere the laser beam impinges on the total reflective surfaces andturning surfaces after the laser beam is introduced through thetransparent material and the incident position is sequentially set ask=1, 2, . . . . Particularly, an angle of the light at which the laserbeam first impinges on the total reflective surfaces or turning surfacesafter the introduction is set to θ1.), the number of reflecting times onthe total reflective surfaces is set to m, and m is expressed by afunction using L, d, nt(λ), ni, φ, and θ1, conditions for at least anyone of L, D, nt(λ), ni, φ, and θ1 are decided so that m is equal to areference set value or more in a range where all of θik are equal to thecritical angle θ or more, thereby introducing the laser beam from saidintroducing surface.
 9. The method of inspecting the ununiformity of thetransparent material according to any one of clams 1 to 8, characterizedin that said total reflective surfaces and said turning surfaces havesuch a relation as to cross at right angles to each other.
 10. Themethod of inspecting the ununiformity of the transparent materialaccording to any one of claims 1 to 9, characterized in that in theinspecting region of the transparent material sandwiched by said onepair of total reflective surfaces and said one pair of turning surfaces,the ununiformity on one certain plane in said inspecting region filledwith light by propagating the laser beam in said transparent material isinspected, after that, said inspected plane is relatively moved in thedirection in which the inspecting region is filled with the light forsaid transparent material, thereby inspecting the ununiformity of theinspecting region.
 11. A method of inspecting an ununiformity of atransparent material by introducing a laser beam through the transparentmaterial, characterized in that the surface of said transparent materialhas: at least one pair of main surfaces which are parallel to eachother; at least one pair of end surfaces which cross said main surfacesat right angles; and chamfer portions sandwiched by said main surfacesand said end surfaces, when an optical path in said transparent materialis uniform, the laser beam is introduced in such a manner that the laserbeam which propagates in the transparent material and impinges on saidmain surfaces and said end surfaces is propagated so as to totallyreflect and repeat between said at least said one pair of end surfacesand is spread in an inspecting region which is formed by the propagationand is surrounded by said main surfaces and said end surfaces, when anununiform portion exists in the optical path of the laser beam which isintroduced and propagated through said transparent material, light whichleaks out of said main surfaces and/or said end surfaces is detected,thereby inspecting the ununiformity of the transparent material.
 12. Themethod of inspecting the ununiformity of the transparent materialaccording to claim 11, characterized in that the laser beam isintroduced so that a singular point where the laser beam geometricallyand optically leaks does not substantially exist on said main surfacesand said end surfaces.
 13. The method of Inspecting the ununiformity ofthe transparent material according to claim 11, characterized in thatthe laser beam is introduced so as to emit the introduced laser beaminto only said chamfer portions.
 14. The method of inspecting theununiformity of the transparent material according to claim 11,characterized in that when it is assumed that a refractive index of saidtransparent material for a wavelength k of the laser beam which isintroduced is set to nt, a refractive index of an external medium whichis come into contact with the transparent material is set to ni, and anangle of the light incident on said main surfaces on which the laserbeam first impinges after it enters the transparent material is set toθ1, the laser beam is introduced so that θ1 is equal to a critical angleθ expressed by sin θ=ni/nt or more in said main surfaces and (90°−θ1) isequal to said critical angle θ expressed by the above equation or morein said end surfaces.
 15. The method of inspecting the ununiformity ofthe transparent material according to any one of claims 11 to 14,characterized in that at least said chamfer portions aremirror-polished.
 16. The method of inspecting the ununiformity of thetransparent material according to claim 15, characterized in that thewhole surface of said transparent material is mirror-polished.
 17. Themethod of inspecting the uniformity of the transparent materialaccording to any one of claims 11 to 16, characterized in that in theinspecting region of the transparent material sandwiched by said onepair of main surfaces and said one pair of end surfaces, theununiformity on one certain plane in said inspecting region filled withlight by propagating the laser beam in said transparent material isinspected, after that, said inspected plane is relatively moved in thedirection in which the inspecting region is filled with the light forsaid transparent material, thereby inspecting the ununiformity of theinspecting region.
 18. The method of inspecting the ununiformity of thetransparent material according to any one of claims 1 to 17,characterized in that said transparent material is made of glass. 19.The method of inspecting the ununiformity of the transparent materialaccording to claim 18, characterized in that said transparent materialis a glass substrate for an electronic device or a glass substrate foran information recording medium.
 20. An apparatus for inspecting anununiformity of a transparent material by introducing a laser beamthrough the transparent material, characterized by comprising:illuminating means for introducing the laser beam through saidtransparent material; and detecting means for detecting light whichleaks out of said transparent material, wherein said transparentmaterial has an introducing surface to introduce the laser beam throughsaid transparent material and at least two pairs of surfaces on whichthe introduced laser beam repeats the total reflection and which faceeach other and said illuminating means is arranged so that the laserbeam emitted from the illuminating means is introduced from saidintroducing surface and, when an optical path in said transparentmaterial is optically uniform, light which propagates in the transparentmaterial and impinges on said surfaces is propagated so as to totallyreflect and repeat on at least one pair of surfaces among said surfacesand the laser beam is spread in an inspecting region which is formed bythe propagation and is surrounded by at least said two pairs ofsurfaces.
 21. The apparatus for inspecting the ununiformity of thetransparent material according to claim 20, characterized in that angleadjusting means for changing an incident angle of the light to saidtransparent material is provided for said illuminating means.
 22. Theapparatus for inspecting the ununiformity of the transparent materialaccording to claim 20 or 21, characterized by further having movingmeans for moving an incident position of the light to said transparentmaterial.
 23. The apparatus for inspecting the ununiformity of thetransparent material according to any one of claims 20 to 22,characterized in that said transparent material and said detecting meansare integratedly and relatively moved for said illuminating means. 24.The apparatus for inspecting the ununiformity of the transparentmaterial according to any one of claims 20 to 23, characterized in thatsaid detecting means has an image pickup camera having an image pickupdevice and a lens for forming, as an image onto said image pickupcamera, the light that leaks out of the transparent material andrelatively moves said image pickup camera and/or said lens in the depthdirection for the transparent material.
 25. The apparatus for inspectingthe ununiformity of the transparent material according to any one ofclaims 20 to 24, characterized by further having discriminating meansfor discriminating the presence or absence, kind, and size of theununiformity of the transparent material on the basis of informationdetected by said detecting means.
 26. A method of selecting atransparent material, characterized by comprising the steps of:preparing a transparent substrate having an introducing surface tointroduce a laser beam, at least one pair of main surfaces on which theintroduced laser beam repeats the total reflection and which face eachother, and at least one pair of end surfaces arranged so as to face eachother in the progressing direction of the laser beam which repeats thetotal reflection between said main surfaces and progresses; introducingthe laser beam from said introducing surface in such a manner that whenan optical path in said transparent substrate is optically uniform, thelaser beam which propagates in the transparent substrate and impinges onsaid main surfaces and said end surfaces is propagated so as to totallyreflect and repeat between at least said one pair of end surfaces andthe laser beam is spread in an inspecting region which is formed by thepropagation and is surrounded by said main surfaces and said endsurfaces; detecting the light which leaks out of said main surfacesand/or said end surfaces without totally reflecting; and comparing saiddetected information with previously accumulated informationcorresponding to the presence or absence, kind, and size of theununiformity existing in the transparent substrate, thereby selectingthe transparent substrate.